An important tip from one of our users: don't just check total power output, find out what you need in the way of current on each power rail. If you need 20 A on your +12V rails and the supply only has 15A, it doesn't matter how many watts the supply puts out: don't buy it. Make certain that you have ENOUGH rails, too: sometimes, you need more than just two +12V rails for your setup.
Replacing an existing power supply is easier: just get a replacement that meets or exceeds the specifications of the old one. (You can usually find those printed on the side of the supply.)
That said, here's a power supply that is compatible with most ATX format motherboards, puts out 600 watts of power (see specifications for exact distribution). Could this be your next power supply?
ATX12V v2.01 CompatibleCompatible with the ATX12V v2.01 standard, the COOLER MASTER eXtreme Power RP-600-PCAR power supply provides enhanced power stability to drive Intel and AMD processors as well as other system components.
600W Maximum PowerThe COOLER MASTER eXtreme Power RP-600-PCAR provides a maximum power of 600W to deliver safe, reliable output for high-end computer systems.
Quiet OperationAn intelligent 120mm sleeve bearing fan delivers whisper-quiet cooling and ensures quiet operation through fan speed adjustments in response to load conditions.