SLi takes off on the X58 platform with its massive bandwidth, particularly Tri-SLi. See below for a lot of multi-GPU.
Personally, I'd run manually overclocked GTX 260 SLi, or if you want an edge grab 216SP 260s.
Below shows you what GTX 260s do vs the HD 4870 1Gb. Results are overclocked and stock for the 216 260 and stock for the 192SP 260.
My Evga GTX 260 is the second GTX 260 I've owned and it is also capable of 730 on the core with a 1532 shader clock like my XFX.
A GTX 260 @ 660 or great on the core should be faster than anything ATi has to offer in single GPU and in SLi it will take 'em in multi-GPU. In FC2 they win @ stock.
If I was buying multi-GPU I'd go manually overclocked GTX 260s. For single the GTX 260 or if you want to get even closer to GTX 280 performance for a much lower price, the GTX 260 216SP.
GPU PhysX in many upcoming titles is another tasty thing to add to the mix.