I actually have two power supplies hooked up to my rig at the moment. This occurred because I bought an X1950 Pro card, which is notoriously a power hog. Along with it I bought a 550watt PSU, and it turned out even that wasn't enough to run the card and the rest of my rig.
So I have the 550 watt powering my motherboard and the X1950 pro, and the stock 350 watt running my disc drives, sound card and Agiea physics card.
A dual power sytem works great, you just have to make sure the mobo and the vid card are on the same source, or it doesn't work.
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