Ahh, Now I see where your going at. The cheapest 260GTX is $240. Getting two in SLI would equal $480 and you would get a 3DMark Score of about 13K-14K. I agree that it could be a better deal, but That just means more Heat pouring out of the case with the two 260GTX's.
I spent a hell of a lot of time looking for a benchmark with a GTX 260 in SLI but I can find one for my life. T_T
And the 4870X2 is already figured into the price. Your really talking about a $70 difference here, but if you want to nickle and dime, You could go with a 260GTX SLI.
As far as I know, having two cards doesn't mean double the performance.
I'd stick with the 4870x2.