LOL... you don't need 550W. 400W is good enough for your rig, unless you're planning to add 3 more hard drives and SLI. But if you want a 500W, the suggestion RichPLS made is a good one.
I'm running 2x 7800GTX OC in SLI, 2X WD Raptors, 1x 4200+ X2, 2x DVD/CDRW drives, 1GB RAM, and like 3 USB devices on a 485W power supply no problem.
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