Correct that the "ViP" series receivers (including the 622/722) have only digital tuners. The antenna you currently have should still work for you with some caveats. The digital channels are usually on a different frequency, most often in the UHF band chs 14-59, vs. the analog channels, so your system needs to be capable of a greater frequency range. To see what analog and digital channels are available in your area, along with a whole bunch of additional information (bearing, range, antenna height, radiated power, etc.) check
AntennaWeb.org .
You can probably use just a simple (passive) splitter to send TV signals to both the TV and the receiver. Assuming your TV has an analog tuner, then that is your obvious option for receiving those analog channels that you still want until Feb. '09 when all analog OTA channels "go dark". One possible consideration: a passive splitter will impart about a 3.5dB loss to the signals going both ways. If your signal strength is marginal, that might be too much additional loss. The analog signals will have additional "snow"/noise and the digital signals will have pixelations/drop-outs, or not work at all. In that case you will need some kind of distribution amp.
You may find out that the digital OTA tuner in your TV is superior to the one in your sat. receiver. That was certainly the case with my Sony XBR TVs vs. the tuner in my 622. I'm in a very marginal OTA signal area, so that difference is noticeable. You may not have the same situation.
To get your Detroit local channels via Dish, you could "move". That's a trick many folks use to get different local service from one area to a "remote" system they own but receive the bill at their main residence, when in fact they are one in the same. Search that topic ("moving") in this site if you're interested in exploring it more.
Oh, one other thought: comparing the HDD size vs. recording time in the 625 vs. the 722 is not an exact science. Tho' the HDD in the 722 has much more capacity, its actual recording time could be considerably less than that of the 625 as HD programs gobble-up that space at a much higher rate vs. SD. You won't have any control over that rate unless an HD program is also available in SD on another channel (often is) and you record only the SD version. Of course the 625 records SD only.
Good luck with it all...!