Yes the 8800 GTX is great, but the price is not and the Ultra version is even more overpriced. For playing games at low resolution, or not higher than 1680x1050, the GTS version would be fine but remember future games will be more demanding...
Hopefully nVidia would releases a new GPUs at the end of 2007 so the current 8800GTX will drop in price.
For gaming purposes, both E6750 or E6850 are very good choice. They're good for overclocking too. Heck even the new quad Q6600 with G0 stepping is great overclocker and probably a little more future proof in next couples years when more multicores games become available. Then again, by that time maybe a newer, better and cheaper multicores processors also start coming out. So, you can choose E6750 or E6850, or even Q6600 as they're all great for the prices now, especially E6750...