As far as the display is concerned, unless you are really into mounting the TV on the wall I would say go with a DLP, LCD, or LCoS over the plasma. You will be able to to save a few thousand dollars at that size. Room brightness and viewing angle also come into play, but unless someone is going to be significantly off to the side of the display, you will be good with the DLPs.
Also, both Samsung and Mits came out with 1080p DLPs this year, so the older models (720p) are cheaper if you can find them.
Check out the Yamaha RX-V2600 as an alternative to the Denon. I think they are pretty similar in price, and the signal processing on the Yamaha as gotten great reviews here on Audioholics.
If you have a Tweeter retailer in the area, the 50" 1080p DLPs are at $3500 (the Pioneer elite Plasmas are $6500 for the same size/resolution). I would also recommend you seriously look into ISF callibration and include that in your budget. It is really amazing how much better the sets look callibrated.
Good luck!