Having a 400W or higher power supply doesn't mean anything if the power provided isn't reliable. Most of the time, the big numbers thrown at us are theoretical peak voltages, and rarely do we see such performance in real-world situations. If you browse user forums where people are struggling with getting a piece of hardware to run reliably, in many cases, the power supply is the culprit.
Let's make this clear, just because you have a generic 400W PSU does not mean you'll be able to get all that power. Unfortunently, even the big companies who preach about getting beefier PSUs for their hardware don't make it clear exactly how much peak, and continuous power you really need. That is why we've always suggested sticking with brand name PSUs, and to read several reviews (including our own :P) where you can gather as much information as you can before making the plunge.
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Review:
Here