In and of itself PCIE 3.0 brings only one real benefit to graphics cards: a effective doubling of the bit transfer rate from 5gtps to 8gtps. (yeah I know: 5+5=10. please feel free to google it.) However, like Intel and their Thunderbolt connector providing additional bandwidth for any data encourages manufacturers to engineer products that will utilize the available resources.
NVidia recently had this to say
. Maybe I'm drawing he wrong conclusions, but with the Intel's latest revolution in transistor tech and the shrink to 22nm in the die process coupled with the predictions made in the aforementioned article, I foresee that trying to futureproof is stupid (nearly always has been). It's not that far-fetched to predict photo-realistic real-time rendering in gaming by 2015.
In the meantime, with the anticipated aggressive progression of GPU tech, going with a 580's in SLI instead of 2 6970's is like arguing pepsi or coke. It really will make very little difference before both are obsolete. And for the short time they are top of the ant hill, neither of them features a title that one setup renders playable while the other does not.
tl;dr. Stick with you current GPU. /thread