The first thing you need to do is look at the specifications of the motherboard. Make sure it has a PCI-e x16 slot because many of the cheaper brandname PC with integrated graphics do not have dedicated GPU ports.
If the PC does have a PCI-e x16 slot then the 7600GT should work. At stock speed (either 400MHz or 450MHz Core), it consumes only 35.8w. There are many brands though that sells overclocked versions of thr 7600GT and naturally consume a little more power.
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ATI cards traditionally consume more power than nVidia cards.