Coming sometime between Nov. 15th and, well.... sometime after Nov. 15th.
It will retail for $999. Don't expect to find it much (if any) lower than that, as dealer cost has been rumored at $950 or so.
As for interaction with your other receivers, it won't really matter much - but you'll now need 5 outputs if you're keeping the other three receivers online (I'm assuming you have an SW44 or SW64 now to feed your three receivers?)
If you're interested in HD (and why wouldn't you be if you're looking to buy a 921), then you're going to want the SuperDish to get the HD channels on 105°. This will require a DP34 switch (included with the SuperDish). Your 5000 will need a legacy adapter to work with the DP34 switch, and you're going to need two DP34 switches (cascaded together) to feed all 5 tuners.
Can you ditch your 5000? If so, you can have your 501, 508, and 921, all being fed from the SuperDish and SW34 switch.