Both are great cards - you'll be happy with either. But I would look at the 6950 1gb, you wont use more than that with 1 monitor. At the prices you're looking at, I think the 6950 is the best value. I believe pretty much any quality power supply that has 600W will be fine, I used to run my 6870 1gb from a 5 y/o enermax 475w.
Enermax, corsair, coolermaster... many other quality brands but that's what I tend to pick from.