The plasma, w/cable card will have a digital audio output which will allow your plasma to directly receive digital cable, then transfer the audio, digitally, to the receiver, so you get full surround with digital clairty.
Big thumbs up for audio and the 2600.
The rest of the stuff is your own future usage of the receiver.
DVD w/HDMI?
Blu-Ray Disc or HD-DVD at some point?
Next generation video gaming?
XM Radio?
As others said, the 2500 isn't a shabby receiver, and if you want to really get some killer audio with the switching the 2500 offers, then perhaps using external amplifiers with it can take you to a whole new level as well.
You definitely aren't looking at BAD equipment - so, I would definitely say you are making some excellent choices if you were to buy today those pieces.
Keep in mind that your speakers and room acoustics play more into the sound of your audio than the receiver does, so the new receiver may not give you a huge jump in sound quality... But, that highly depends on your existing speakers and room equipment and setup.