My old B&W G3 is running 24/7 with ATX PSU (350W) for few years and I have no issue copying gigs of files over a 2.5" external drive powered by firewire.

I have also done it on Sawtooth. 400W ATX PSU. The only thing you need to watch is when you add faster proc and power hungry graphic card (for example ATi Radeon 9800).