they both have the same size frame buffer, but a common misconception that people have with graphics cards is that the performance is solely dependent on the amount of memory it has. what really determines performance is amount of memory, the number of processing cores it has, the clock of those cores, and the GPU (graphics processing unit). but it's easy to tell that the 8800GT is faster than the 8600, likewise the 8800GTS is faster than the 8800GT, and the 8800GTX is the fastest of the 8000 series, the 9800 GX2 is faster than all of them, the GTX 260 is the second fastest, and the GTX 280 is the fastest you can get, it's huge, uses a lot of power, and kicks out a lot of heat though. the 8800GT is cheap, but it's performance is hardly third tier, thats why it's such a popular card. remember, the 8800GT should be your minimum, the 8600 would work all right, but not very well. but if you plan on playing crysis....then Id get something like an 8800 GTX, or a 9800 GX2, since the GTX 280 came out, they've become more affordable, and since ATI came out with a series that rivals everything nvidia has, now they're pretty cheap. a 9800 GX2 should cost about 300 bucks, a GTX 260, $330.
if you're looking for ATI cards that have similar performance, then this might help
4850=8800gt (except for a performance drop in unreal tournament)
4870=everything else except for GTX 260 which has a razor thin advantage, and maybe the 9800GX2 (which scales pretty poorly)
4870x2<GTX 280 (but it has other advantages such as efficiency, and it scales very well in games such as bioshock.
to all those who say Im spewing crap, look at the benchmarks first, then pass your judgements.
AND FOR CRYSIS ID GET AT LEAST A 9800 GTX, OR A RADEON 4870. MAKE NO MISTAKE, THAT GAME WILL BRING MOST COMPUTERS TO THEIR KNEES.