I'm guessing about 150 watts. About 20-30 watts for the motherboard, 70-80 watts for the CPU, 5-10 watts for the HD, a couple of more watts for the memory and a bit of surplus.
This should probably do nicely.
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Riocar 80gig S/N : 010101580 red
Riocar 80gig (010102106) - backup