As I recall from the GPU benchmarks, these two chips are pretty close in performance. That allows you to look at other criteria. Is this for gaming? I notice at gpureview that the Radeon is DirectX 10.1 compliant and GeForce is 10.0. Will you be gaming in Vista? If so, you might want the 10.1.
From looking at the other thread, I'm not sure exactly which motherboard you are considering, but most of the Gigabyte ones are CrossFireX compliant and fewer support SLI. Do you think you might ever get a second board for more gaming power? Then you should have a CrossFireX board for dual ATI cards and an SLI board for dual Geforce cards.
As for the power supply, you should probably pick one that is approved by the graphics card maker. These cards will be the most power hungry devices in your system, so they should have plenty of clean power. Also, if you are considering a dual config at some point, ATI and NVidia both have a list of power supplies that are approved for use in that case.