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I have seen a 6600GT work fine on a PC with a 250W PSU. The problems start happening when someone plays a hardcore game like Doom 3 with max settings AA and AF and for a long time. I suspect those power consumption readings don't consider length of time because once the fans start going and etc I would think more and more wattage is used. Also, most people think just because they see 400W on their PSU they think it is constant but it could mean peak. Anyways, upgrading a 250W PSU is always a good option because it improves stability and ensures less chance of your PSU stressing out.