The 8800GTS 512 is definetly the one you want.
And forget about the cards you saw with 1 gig of memory.
Over 512 meg with a 256bit wide memory buss does NOTHING for you performance wise.
That is why the old king of GPU's, the 8800GTX has a 384 bit wide memory buss to go with it's 768 meg of memory, and still kicks every other cards butt at high res with AA enable. To utilize the extra memory without a bottleneck, you need a wider memory buss.
The 1 gig cards are simply a marketing ploy to entice people into thinking more memory is better, without researching what the card's other specifications need to be in order to make more memory actually useful.