The last thing you want to skimp on when building a rig is the PSU, it's the heart of any system. And don't buy into the recent hype about needing 700W+ PSUs. A quality PSU in the 450W+ range will run almost anything short of a top of the line SLI rig w/ all the bells and whistles. In other words, when it comes to PSUs, look for quality over quantity i.e. Antec, Corsair, Fortron (FSP) Seasonic, etc. etc. You can find a quality PSU from these and other companies that'll fit your needs for <$100 and believe me, it's money well spent.
