2 GTX460's are pretty decent but if you go GTX470's, its obviously better. BTW - have picked the cheapest GTX470, its a stardard, non factory overclocked model. But these are pretty awesome cards.
GTX 470 ($259.99) and free shipping (this comes with Mafia 2 downlaod free, and "lifetime" warranty)http://www.newegg.com/Product/Product.aspx?Item=N82E16814130550
I have the ASUS reference cards (Very similar to the above, just different logo on the card) and they run everything very well at the highest settings.
Mafia 2, Medal Of Honor, Just Cause 2, Fallout New Vegas, Sigularity, Wolfenstien, Assassins Creed II - etc etc etc, so many games, so little time