I'm not sure the GTX480 will fit in the 300. You should double check that.
I think Nvidia says you need a 500 or 600W with around 40A for a system with the GTX480. I know the Antec Neo 520 has 40A, so most quality 550W PSUs should be enough. The TDP for a GTX480 is around 300W, so if you want an SLI rig you should plan on a 750 at least, an 800W is a better idea.