Before you choose , you must know its all depends on Monitor
you should ask your self , what Resolution you use on Playing games ?!
Dual Card are awesome on PhysX® technology , High bandwidth means the ability to handle large textures and anti-aliasing and anisotropic filtering as well (Buy 1GB Ver.)
its also use Less power than 480
considering buying GTX 480 isn't bad choose at all its will help on long Term in case you wanted to Dual GTX 480
and by the way GTX 580 is out on Market so think twice before you do your move
GTX 460's scale to around 95% - meaning you almost get double the performance ( Obviously depends on the game ) But with ATI cards the CFX doesn't scale as well, adding a second card may get you a 40% max increase compared to SLI's 90% range. I lost the posts that proves this but I'm sure they can be found on Google.