Posting frames per second and benchmarks of one card is not good, because it may not be true for him, comparisons are better.
Go with the 6600GT. You will be happy with it. ForceWare is, in my experience, strongly superior to Catalyst drivers, and the hardware is better-supported. Additionally, the 6 series are better designed overall than ATi's competing cards.
I must say this. Memory amount is not everything. The average consumer might see a card and say "ooh, 256MB, it's obviously a better choice." That is not true; the important thing is the chipset you're using. And Nvidia totally wins there.
As for which 6600GT to get, I recommend the BFGTech model. According to [H]ard|OCP, it averaged about 47 fps on Doom 3, 43 fps on FarCry, and 81 fps on Half-Life 2. That's pretty impressive.
The review benchmarks for the 6600GT vs. the X800GT:
Doom 3: 18.1fps
3Dmark03 G2: 12.9fps
HQV score: 73
Doom 3: 18.3fps
3Dmark03 G2: 12.2fps
3Dmark03 G4: 23.0fps
HQV score: 55
Note: This is a comparison between a X800Gt not GTO and a 6600GT. So a GTO with four more pixel pipelines, should definetely outperform a 6600GT.
As far as software and drivers go ATi has been more helpful performance wise.