Getting another 460 would be your best bet, there is no reason for you to go out and spend a crapload of money when you would get about the same performance from a 580 as you would get from two 460's. the only time time the 580 would really beat two 460's is if you use more than 8x anti-aliasing and in my opinion, that's not worth spending 500 bucks compered to just spending about 150 for another 460.