Either way you go it really doesn't matter. I've gotten 3 years out of my single Gtx 260, and havn't had any issues with FPS. I know I'm going to have to upgrade to get the best experience for CoD, but damn... I was gunna go the route of a single Gtx 570 or something.

The 580s would probably be better than the 5870s but at the same time a bit more expensive. I'm curious as to how much these rigs cost. Any word?
How about you wait for the Gtx 590s to come out and go triple SLI?! You'll get at least 10 years out of that hahahah only kidding
P.S. - Sorry i've been drinking.