My 600W Enermax set me back a fraction under £100, and it's worth every penny......powers the 6800GT, two hard-drives, two optical drives, as many USB devices as I want to plug in (except USB hard-drives, thanks to the motherboard - avoid VIA chipsets, at all costs - a 500GB firewire drive was fine, but plug a 40GB USB drive in and Windows decides it doesn't like running at all), a printer, etc etc...

Definitely don't skimp when it comes to power supplies...