Well of course two GTX 460's would out perform the 5970... The 5970 would eat one 460 up so just get one 5970 and save money.....
But at least you're recommending an Nvidia product to someone who has some nice cash. I myself go with ATI since I can't afford any Ferarri's like the rest of you and with the way Nvidia likes to put their prices ;D (Please don't get offended by that... I'm being sarcastic and at the same time i'm not)