About 1.5 months ago, I got a 921. I had Dish 500 Twin, pointing at 110/119, and a Dish 300 pointing at 61.5. I had 1 SW21, and a Dish 6000. I had a fairly similar setup to what you are referencing, although with fewer connections.
I did a lot of investigation as to minimum requirements, as well as costs, recommendations from better informed people than myself, and I scoured the Internet. Here is what I decided to do:
1) I decided that the upgrade cost of going to DishPro was worth it, rather than continuing to try to use Legacy Dish LNBF's. I went to Ebay & bought a DishPro Twin for ~$50 including S+H. I also bought a DishPro Single for ~$20 including S+H. I bought an SW34/DP34 switch for ~$70, including S+H. If I had to do it again, I would buy a DP44, which will support 4 Satellite inputs...not just 3. Case in point, if you ever want to get locals over satellite, think Super Dish (105, 110, 119, or 110, 119, 121 - either way, 3 satellite inputs) + 61.5 (maybe) = 4 inputs. Or, I would buy a DishPro Plus switch...more on that below.
2) To move to DishPro, it is highly recommended that you ensure you are running RG6U cable rated to 2150Mhz. At Lowe's Home Improvement (I'm pretty sure you'll have one near you...), Zenith has a display with all the best materials...RG6U quad-shielded (2 layers aluminum braid + 2 layers aluminum wrap), rated 5-2300Mhz (says so right on the cable...). THEY ALSO HAVE COMPRESSION ENDS! Rather than crappy crimp-on F-connector ends, they have a little (crappy) compression tool w/10 ends for ~$25. Good news/Bad news...Although I am a big believer in compression ends, they barely fit in the Dish 500 Twin holder...you know, the plastic arm that connects the steal arm extending out from the actual dish to the LNBF's...I actually broke one of the initial compression ends into 2 pieces while trying to encapsulate it in the plastic connector, and had to redo the end -- but it all does work, and did fit...just very tight.
3) You have to run 2 independent cables from your switch(es) to the back of the 921 -- unless you upgrade all the way to DishPro Plus. I did not, because I believed that the technology was so new that the cost/benefit ratio was not warranted. However, the 921 will support DishProPlus (DPP), and with DPP, you only need 1 cable from the attic to the wall outlet behind your 921, and then you can split the signals to the 2 Sat inputs on the 921. So, if your wiring absolutely cannot be changed to get 2 cables there, DPP would be the only option. If you can get another cable there, then either Legacy (what you have), DP, or DPP are options...but I would suggest at a minimum, go to DP...it will cost you as much to upgrade your SW21's to an SW64 as it will to just upgrade your LNBF's and Switches to DP, and then you can have fewer wires entering your home, and an easier future expansion to more receivers (or 2-tuner receivers).
4) If you are diplexing OTA onto your satellite feeds, let me warn you right now...I tested this personally...the DP34 switch will NOT pass ANY OTA through it! While the SW21's WILL pass OTA through them, the DP34 will NOT...you'll have to diplex the OTA onto the SAT feed between the DP(34/44) switch and the SAT wall outlet; and you'll need to diplex it off the SAT feed between the wall outlet and the 921. I have tested this successfully, but not yet implemented my final implementation (waiting until it's nicer out to put up a REAL antenna on the roof...).
5) What I'm about to outline didn't apply to me, as the only Set-Top-Box (STB) that I previously had was the 6000...so after I changed out to 921 and DP, I sold my 6000 on Ebay. Then, I got on the list with Dish to get an 811 for $149 (to effectively replace the 6000...but with DP-capable technology). In your case, if you keep the 4900...you'll only need the 1 sat line to it ( must also be RG6U rated for 2150Mhz), but, you'll need an ~$60-70 DishPro/Legacy adapter (http://www.switchinfo.info/dishpro.html). My suggestion would be to also consider replacing the 4900 with a newer DP/DPP-enabled receiver, and if you don't need a backup Hi-Def STB (in case the 921 fails or needs service -- the real reason I ordered the 811), then I would suggest another DVR...once you get used to having that functionality on the 921, you'll be unhappy not having it everywhere.
So, in my case, I have 3 wires coming through my roof (my dishes are separately grounded), to the DP34 in my attic. I then pulled 2 new lines from the DP34 to a wall outlet behind my 921. I then pulled 1 new line to an outlet in my bedroom (to where the 811 will go). I then made new 6' cables with the fancy RG6U cable and compression ends to connect the wall outlets to the 921 and 811.
Eventually, I will install an old-fashioned OTA medium-fringe antenna on my roof, run it's line into my attic, where I will amplify, and split. I will then take each split and diplex onto 1 of the 2 lines to the 921, and the line to my bedroom. I will then create 1' RG6U cables to connect my wall outlets to a diplexer and another 6' cable per system to connect the diplexer to both the STB and the OTA connector on the 921/811.
NOTE: I read several places where there may be some problems with passing amplified OTA signals diplexed on the same line as the sat feed. I do not remember all the issues and relationships, and it was explicitly stated that if done wrong, this can toast either/both the DP switch and/or the Sat tuner in the STB. Please investigate that more before Diplexing OTA onto your Sat feed.
The following link provides a really good explanation of the difference between Legacy Sat LNBF's, and DP/DPP LNBF's that stack the frequencies, as well as why this is important...it will make the whole conversation make more sense.
What you would need to make it work depends on what your exact Dish/LNBF/Switch configuration is. The information would seem to indicate that you have a dish300 with a legacy dual LNBF pointed at 61.5 and a dish500 with either a legacy TWIN LNBF or a legacy QUAD LNBF connected to your 6000 and 4900 through 2 SW21 switches.
If your dish500 is equipped with a legacy QUAD LNBF the minimum you would need to get is a SW64 switch ($134.00 from dishstore.net) 2 additional runs of RG-6 between the dish500 and the switch and a second run of RG-6 coax to the location of the 921.
Alternately you could convert to a DishPro LNBF/Switch setup. This involve replacing the DUAL LNBF on your dish300 with a DishPro SINGLE LNBF ($47.99 from dishstore.net) replace the legacy TWIN or QUAD LNBF on your dish500 with a DishPro TWIN LNBF ($39.99 from dishstore.net) and replace the SW21s with a DP34 switch ($112.00 from dishstore.net) You would still need an additional run of RG-6 coax from the switch location to the location of the 921 but would need only 3 runs of RG-6 coax between the switch and the dishes (2 runs to the dish500 only 1 needed to the dish300) Neither of your current receivers is a DishPro unit so you would need to purchase a DishPro Legacy Adapter (]$65.00 each from dishstore.net) for each receiver you wish to continue to use.
If you wait long enough there is a new switch scheduled to come out this year that you could use. It it the DishProPlus 44 switch. You would still need to replace your legacy LNBFs with DishPro LNBFs as above but would use the DP+44 switch instead of the DP34. The DP+44 supports legacy receivers and you would not need to purchase DishPro Legacy Adapters for your current receivers. Also the DP+44 can feed both tuners of the 921 over a single run of RG-6 coax if you use a DishProPlus diplexer. The DP+44 is currently expected to ship in the may 2004 timeframe but I have not heard an estimated cost for it or for the DP+ diplexer
I had the same setup except I had a 301 and a 6000. It took me a while to get the proper advice that is included above, but eventually I figured out that the best way to go is to go Dishpro. I am replacing my only legacy receiver, the 6000 with a 921 so I don't need the adapter. I got a dp twin for $29 on ebay and a dp34 for $74, shipping included on both. I'm still waiting on a good price for the dp single, but I think I have a few weeks to be patient since they're not shipping 921's yet anyway. I hear maybe the week after next. Maybe!!!
I have a dishpro quad lnb I bought for $45 and am about to replace the 2 single lnb's on my dish. But I also bought a square shooter antenna so I could get hidef stations over the air. Now.. I need to feed 4 different dishpro receivers with the satellite signal into 3 different locations and then I want to feed the ota antenna to those 3 different locations too. Should I use 3 diplexers to do this? How does a diplexer affect my satellite signal strength or the strength of the hidef ota signal? I have about 90% strength on the single lnbs now thru the combiner switches and very good picture on the hidef ota stations, so I think Im okay. Id like to use some sort of multiport diplexer, but then if I get something like that, that combines 1 or more sat signal and an ota antenna and then feeds 4 or 8 receivers, do I just not use the other 3 connections on the quad LNB or should I just sell that quad lnb and stick with the single lnbs and the sw21 switches?
Im thinking that 3 separate sets of diplexers would be better except that I would then also need a 3 way splitter from the ota antenna to feed the 3 diplexers. Which way should I go?
Something doesn't quite add up. I don't see how you're running 4 receivers off of 2 single LNBs and SW21's. So, let's assume you're expanding your setup.
Unless we're talking satellite feeds out towards the DishPro 200 foot limit, the OTA diplexers won't hurt the satellite signal. Splitting your OTA signal 3 ways might be an issue - but that's easy to fix. Just get an antenna amplifier with enough outputs. They don't cost much and are readily available. Make sure the diplexers are rated for 2GHz on the satellite side.