Putting any bias aside, the 700 series chipsets only strong point is that they support SLI, because the X38/X48 are far superior than them. If you haven't checked the latest benchmarks I suggest you do, because you'll see that the $300 HD 4870 matches/beats the GTX 260 on almost every single test except for oblivion, and on half of them it even closes in to the GTX 280 performance territory.
a 790i board ($350), two GTX 260s ($800), and 4GB DDR3 1333 Ram (~$200).
an X48 board ($230-250), two HD 4870 ($620), and 4GB DDR3 1333 Ram ($200).
This quick comparison shows savings of up to $300 (could be even more if you look hard enough), and instead of sacrificing performance you would be gaining it if you went with the cheaper system. (HD 4870 are all around better cards compared to the GTX 260s and the X48 performs far, FAR better than any nvidia motherboard.)