i would get the 4870X2 out of those two.
Reason is because SLI is not perfected yet, so i think a single card with 2 GPU is better than SLI/CF.
though it would be cool to say i have 2 cards in SLI....
but it will also be cool to say.. "I Have the Best Card Out Right Now Bia**h" when comparing to others. haha
the 2x4870's without question!!...
just like Teny said, SLI isn't perfected yet...and i am pretty sure the 4870's would out perform the 260's...well it'd be a really close comparison i think.
i thought he was refering to the 4870's in xfire. my mistake. but like Teny said, and i couldn't agree more....the 4870x2core would be much better then sli...and can you imagine running that card in xfire...WOW