The quads are nice... the qx6600 is an inexpensive way to get a 3.0ghz quad core. (will clock to 3.2ghz on stock voltages and air cooling, but you'd want a different heatsink than comes with it stock) The only thing to bear in mind is what you will be using it for. Videogames are not being designed for multi-threaded computers yet, or rather, they are only just getting into being dual-core friendly. Most videogames will not take advantage of your quad core, but if you do video/audio editing, you will notice significant improvement in encoding times.
Much as Legendaryfrog pointed out in your thread... the Ultra isn't really worth the extra $200-$300 over a gtx. (this is just my opinion based on having used them minimally and having compared synthetic benchmarks... ultimately which one is worth it is up to you
) Check out the new 8800GTS 512MB cards... they come with a higher clock speed than even the ultra, and are $360 brand new. SLI'd, they'd be blazing fast... and likely beat out a pair of ultras. My buddies and I unofficially recognize it as the 8900, to clear up confusion between the 320/640 and the new 512 mb S model of the 8800gt. I myself am still an AMD fan, though they are being outdone by Intel presently. I'm hoping the new Phenom project isn't a total flop... would be nice to see real quad's made by AMD. Their focus now is more "power per watt" not just "raw power" and I believe that is why Intel is winning out right now... they just want "raw power."
The 850 watt Thermaltake Toughpower will definitely hold under 2 Ultras and basically anything else you can throw at it... VERY stable and reliable psu.
Good luck with your rig. You'll be glad you made the investment.
Again though, very nice setup on the plasma. Must be awesome for watching movies, playing games, and just overall entertainment at parties.