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 620 watts of power (see specifications for exact distribution). Could this be your next power supply?
ATX12V v2.2 and EPS12V CompatibleCompatible with the ATX12V v2.2 and EPS12V standards, the CORSAIR CMPSU-620HX provides maximum power stability to drive the latest Intel and AMD processors as well as other components for desktop and entry-level server computer systems.
620W Maximum PowerThe CORSAIR CMPSU-620HX provides a maximum power of 620W to deliver safe, reliable output for high-end computer systems.
Highly EfficientDelivering up to 80% efficiency, the CORSAIR CMPSU-620HX power supply saves energy and may even reduce your electric bill!
Efficient CoolingA 120mm double ball-bearing fan delivers highly efficient cooling for the power supply and the whole system. The thermally-controlled fan ensures whisper-quiet operation.
Modular Cabling SupportThe CORSAIR CMPSU-620HX features modular cabling support that allows use of only the cables you need resulting in improved internal airflow and reduced system clutter.
NVIDIA SLI CertifiedThe CORSAIR CMPSU-620HX power supply has gone through a series of rigorous NVIDIA SLI certification tests for maximum system stability when building NVIDIA SLI PCs.
1 x 20+4-pin ATX 1 x 4-pin +12V 1 x 8-pin EPS12V 2 x PCI-Express 10 x 4-pin peripheral 2 x 4-pin floppy 8 x SATA
Corsair has engineered the HX620W with features such as whisper quiet thermally controlled 120mm fan, up to 84% high efficiency, combined 50A on triple +12V rails, and modular cable connections with enhanced flexible cables to provide sustained 620W output even at 50ºC to the most demanding of systems. The HX Series is the ideal power supply for high performance and gaming PCs, Audio/Video workstations, and file servers.