If the question is two 9800 GTX+ in SLI vs a single 4870 then, of course, the pair is more performance. I believe a pair of 4870's would outperform a pair of 9800 GTX+'s, though.
buy 4870 now or gtx 275 now and then later you can crossfire them or sli it will be more future proof one card is always better eventhough the gts 250 in sli will perform better than single 4870 but you should go for a single card and in sli you will need 750W PSU
but i will always go for a singl card because its futureproof
While I'll disagree with barney.stinson on the whole "future proofing" thing, Two cards are generally better than one, but a single powerful card is a better choice than two weaker ones in the long run. First of all, they will usually be cheaper (unless you get really outdated cards) but you can also decide to SLi/Xfire them later, for an added (usually large) boost in performance for a lower price seeing as price drops on cards over time. Another benefit is that running two cards or more usually draws a significant amount more power than even a single beastly card. I personally would go with the 4870 (or maybe even 4890) and Xfire later on. But to each their own, SLi/Xfire makes for excellent bragging rights