In most games,2 460's will performs as fast a 580(or even faster in some games)
http://www.hardocp.com/article/2010/12/02/geforce_gtx_580_vs_460_1gb_sli
"What strikes us about our gameplay experiences today is the fact that with GTX 460 1GB SLI the 1GB of RAM on the GTX 460 may be holding it back at higher resolutions with AA. In all of our gameplay testing, we were surprised how well GTX 460 1GB SLI keeps up with the more expensive GeForce GTX 580. While it comes close, and at times exceeds it in framerate, it doesn’t deliver the same gameplay experience. The GeForce GTX 580 has more RAM on board, 1.5GB to be exact, and this seems to be giving it an advantage in some DX11 games when AA is enabled at high levels."
"GeForce GTX 460 1GB SLI offers performance that competes with a single GeForce GTX 580, but ultimately is limited by its 1GB of memory per GPU at high resolutions with AA. The GeForce GTX 580 offers a smoother gameplay experience and higher playable settings. The GeForce GTX 580 may be more expensive, but that price can pay off as we move into new DX11 games pushing the boundaries of graphics card memory capacity. The fact is GeForce GTX 580 offers you more options by allowing you to install a second video card that improves performance even more."