Re: Which of these three Nvidia 8800's is the best option?
Crysis is way beyond any other game in terms of requirements. To max out Crysis at high mode (not even very high DX10) with AA and AF, 1440 res, you'd need at least an Ultra, or possibly an SLI rig.
That said, the GTS 512 or even the GT should play most things at high settings and that resolution, Crysis is just a beast. And really not worth the extra investment, if you want my opinion (I think they've framecapped it to make you buy the new 9-series cards when they come out)
The 640 GTS, even SSC version, is old architecture and outperformed by the GT. The new GTS (512) offers a marginal improvement over the GT at stock, but you can hardly overclock GTs sometimes (depending on how lucky you are when you get yours), as they run so hot, so the new GTS has more overhead if you want to overclock. EVGA are good, they guarantee cards, even if you overclock, and have 90 day 'step up', (rubbish for Europeans though).
As for the GT 256, 1440 res with AA and AF is prolly too much for most high end games, so I would certainly go with the 512. The performance drop by going down to 256 is actually more than anyone expected, mainly due to the fact that people use so much AA and AF nowadays.