Well, considering the number of watts and 12V rail of a cheap Antec, I would shy away unless building a system that won't be very powerful and that I'm never going to upgrade.
In 2 or 3 years, we're going to have quad core processors that may use enough power alone. Then there is the fact Nvidia already made a request to the organization that establishes standards for PCI-E for another 75 watts to PCI-E cards. And we don't know yet how much the Geforce 8 line will require.
I did some research and I'd want at least Seasonic or Fortron 500 watts. Get a cheap Coolermaster Centurion 5 without PSU or Antec SLK3000B without PSU and a Fortron 500 watts.