You'd want to size for a sustained 350W power draw while gaming, which I'd say calls for a 500W to 700W PSU. The combined power at 12V is critical if you are trying to "rightsize" a power supply: the card alone will sink up to 16A at 12V, and the rest of your system will want another 12-14A at 12V. So allowing another 2-4A of headroom, look for a power supply with either a single 12V rail of 32A or better, or a specific rating for the combined 12V rails (don't just add them) of 32A or 384W or better.
Example: A good mid-grade power supply like an Antec Earthwatts 500D is rated 500W maximum continuous; so far, so good. It has two 12V rails at 22A each, but that doesn't add up to 44A: it has a combined rating of 444W at 12V (which works out to 37A). That is sufficient for your proposed rig, but doesn't leave a lot of headroom if you want to upgrade to a Crossfire setup, 5970, or nVidia Fermi later.