Gtx 260 vs 4870 for new gaming pc

First of all, so sorry for creating a 'help me' topic, but I'm drawing a blank here.

First I was considering 9600 gt in sli, then the 9800gtx+, which turned into the gx2, which I decided costs too much. So I settled on the Gtx 260. Not terribly expensive, got a rebate and everything, and the possibility of running it in sli further down the road.

Was talking with a relative that knows hardware way better than I do, and who's opinion I trust, and he recommended the 4870 in a way that was to say there was no competition. I didn't see that much of a difference, but I'm not any kind of informed person.

Wondering which is the better card, and worried about getting a reasonably priced motherboard that will allow me to run whichever in sli or xfire