I think you could get by without a dedicated power amp, but you're going to want more muscle than the RX-V2500 has to offer, especially if you go with less sensative speakers.
Denon's AVR-5805 is the receiver to end all receivers. It has ten channels, which means it can run two independent 5.1 systems at once, or one monster 10 channel theater with two centers and six surround speakers. Receivers dont come any bigger than the 5805...or more expensive. Its got a 6 grand price tag.
The Denon AVR-4806 and Marantz SR-9600 cost a little more than half of that, and still pack 140W x 7, iLink (invaluable for SACD and DVD-A) and HDMI switching, I suggest taking a look at both. I think both Denon and Marantz do a much better job with music than Yamaha. Pioneer's 59TXi has the muscle, but no HDMI.
As for speakers, I think Definitive Tech's BP7000SC power-towers are unmatched for home theater use, and provide incredible bang for the buck, considering each one essentially has a Definitive Supercube subwoofer built in to the cabinet. They'll definitely have no trouble filling a room of that size.
The C\L\R 2000 is the passive center channel that goes with the 7000s, or if you're nuts and two 14" subwoofers, four 14" passive radiators, and two 1800W class D amps arent enough bass, the C\L\R 3000 has its own 10" 150W powered sub. As for surrounds, you could use a C\L\R turned vertical, a Studio 450 for a monopolar surround, or a BPVX/P for a bipolar surround.
For a TV, I suggest Toshiba's 72HM195. 72", 1920x1080 resolution, just 5 grand. Thats the price of a 50" 1280x720 plasma. Samsung has a 71" HLR-7178W, also 1080p, but it costs $1000 more than the Toshiba, and I dont think it looks any better.
If you dont have one already, you will want to invest in a DVD player with HDMI upconversion. Dont spend too much money on it though, because hi-def discs are due soon. A Toshiba SD-4980 will do the job for just $100.