By the time I've posted this, you probably have already chosen what setup you'll have. But here's my two bits anyway.
Well, I do like the products that you've thought of so far. I hope you don't mind a few suggestions. I've also been struggling with which graphics card to choose exactly like you. I want the best performance I can get for my money since I am on a budget. I looked at the GTX 570 and it seems to be worth the money more so than the GTX 580. There is roughly a 120+ dollar difference.
Here is a website (or two) to help show the performance of the GTX 570 in comparison of the GTX 580. (you'll need to copy and paste)http://www.guru3d.com/article/geforce-gtx-570-review/1http://www.geforce.com/#/Hardware/GPUs/geforce-gtx-570/performance
The 6950 and 6970 both provide good performance on a slightly smaller budget. But price to performance is about the same when you consider the GTX 570 into the whole lot. (here's another website)http://www.guru3d.com/article/radeon-6950-6970-review/
The 6950 performs slightly slower if not the same as the GTX 570, whereas the 6970 performs better if not equal to the GTX 570.
There is another really good website for comparing graphics cards.
They have a tool that allows you to examine different benchmarks and compare the results. It is called "bench". Currently 5:41pm May 30, 2011. That section of the website is down, but "should" be back up and running soon.
I hope that these help.
p.s. after reading some of these replies I might go to a i5 2500k instead of the i7 2600k. Good luck.