Check the power supply label. You need a total of 40 amps or more on all the 12v rails combined (add them all together). If possible, lift up the ps; it should weigh 3.5-5 lbs. Lightweight units use cheaper components. I like antec, enermax, pc power and cooling, seasonic, ocz, and corsair.
I mostly agress with oldie, but you don't really need more then 30A on the 12V rail unless you want to go CF/SLI. This means any quality 400+ PSU will be all you need. Most people buy a PSU based of the connections they need, and the CPU + GPU they will be running. If your motherboard will be using a 24pin ATX bundle as well as an 8pin aux, then get a PSU that supports that. If your running two GTX280 video cards in SLI, get one with enough 6pin PCIe connectors, and enough watts/amps to run both cards together.