you know what, i'm not quite sure, either. i mean, your logic is optimistic, but not off base. i wouldn't expect 2x performance, but you are totally correct in saying 10% is not right.
apparently there are different SLI modes, where the cards work together to do different things. there should be more than 10% improvement, but 2 isn't always better than one. for example, 2-core processors don't work twice as fast. and would you rather have 2 STDs than 1? hah. i think not.
for me, SLI would never be an option, because i couldn't fathom paying for the same card twice. that just seems like a waste of money to me. and the speed at which the graphics market moves makes it seem even more dumb to me.
i did some research on the topic, because i wanted to know, too, and i didn't want to wait until someone else came by and answered. wikipedia wasn't much help, but i found an interesting article here on Tom's from when SLI first came out (
Link! ). apparently CPUs have never had enough juice to keep a SLI setup well fed enough to see sizable benefits. so, you basically have to play at very high rez or... play at very high rez to see a benefit from SLI.
i never knew why it wasn't 2x as fast, and i kinda do now, but it still seems like an obscene waste of money to me.
the current dx10 cards should do Crysis FINE, trust me. think about FarCry, it did a similar thing with dx9... it came out very early and played okay on a variety of cards. crysis will be designed FOR those cards, and those cards for crysis, so worst case scenario would be you wait till the game comes out, there are new cards from both companies, and everything is fine.