There are other good choices, but if I were building that rig:
1. If I did not plan to add another GPU, I'd get a 550W Antec Truepower New unless a deal brought the 650W version to within $10-$15, in which case I'd get that.
2. If I were planning to add another GPU, or simply wanted more headroom, I'd get a Seasonic X-650 if the budget allowed, a 650W Truepower New if money were tighter.
Personally, I would get an OCZ StealthXstream II 600W because it's the cheapest 600W PSU from a reliable manufacturer on the website I like to use for parts, ebuyer.com. That would be if I wasn't planning on using dual graphics cards in the future. If I was, I'd take a look at some 650-700W PSUs.