ohh I dunno, one of my workmates has an EVGA overclocked 480, he's overclocked it a bit more, but that's in a HAF X case which has awesome cooling, and the sheer size and heat of it (it burns your fingers after being on for a couple of minutes!), as well as insane power draw puts me well off.
To put it more succinctly, 2x 1gb 460's with a bit of an overclock are going to use less power, generate less heat, and outperform 1 480, for at least 100 dollars less if you don't have an SLI capable motherboard well you can either wait to see what high end GF104 based card Nvidia will bring out, go the ATI option, or buy a new motherboard of course!