What do you have now, in terms of PSU? If you have a current system and are simply adding the GF4 in as an upgrade to an older card you should be fine, but if you're building a new system depending on the processor and other components you intend the plop in 320-350 watts would be a good bet. Don't go cheap on the PSU either, try to one with a solid reputation like Enermax and such. Some cheapo ones say 400 watts, but can barely power components, so again get a decent name brand and you should be good!