i've got 3 x 24" screens in ifinity.
it seems to keep up okay.
i was playing assains creed2, it does'nt lag.
when in ifinity it creates a new resolution.
I did use two 6770's in crossfire, but 1 x 6950 performed better.
maybe 2 x 6870's will do better than my earlier test.
a mate of mine uses 2 x 6850's. he said it works like a boss
hope this helps. cheers, Rolly
side note, read up on flex version of cards, vs display port adapters etc. cheers =)