Things to be looking for, at least 80+ basic certified, and active PFC(no voltage selector switch)
A good 500-550W unit with ~40A on the 12V rail will handle any single GPU easily. I suggest looking at units from Corsair, Seasonic and Antec. Check out some 650W units too, sometimes you can find an Antec EA650 Green or an XFX 650W unit for about the same price which can give you a good upgrade path for the future.
If you ever decide to use a single high end graphics card with dual GPUs like the just announced GeForce GTX 590 or the Radeon HD 6990 you're going to need at least a good name brand power supply unit providing 800 Watts or more.