"The Bottom Line
The HD 5970 delivers faster gaming framerates compared to the GeForce GT 580. However, if you look at the actual usability of those frame the picture is a bit different. The GeForce GTX 580 allows a consistently higher level of the gameplay experience compared to the Radeon HD 5970. We were able to game at higher settings with the GTX 580 than we were with the Radeon HD 5970. The most important factor, beyond framerates, is the visual quality and experience returned by the product. The GeForce GTX 580 allows a more immersive, smoother, and consistent quality of gameplay."
"The largest issue we had was running into a video RAM wall with the Radeon HD 5970. There is 1GB of RAM available to each GPU on the Radeon HD 5970. This is simply not enough for the latest DX11 games."
"The Radeon HD 5970 runs into a wall at 8X MSAA at 2560x1600 with all the effects turned up. Performance becomes erratic, inconsistent, and downright unplayable. at 8X MSAA. However, on the GeForce GTX 580, which has more memory on board (1.5GB), performance is smooth, and most importantly playable at identical settings."
" If you already have a Radeon HD 5970, keep it, it is a great card and you already know that if you own one. If you are on a lesser generation, and looking to upgrade right now, no question, get the GeForce GTX 580 if you have the cash. The extra money spent will pay off with its higher VRAM capacity, and higher level of consistent gameplay experience. Plus, you can always hook up two GTX 580 cards in SLI that will surely blow the 5970 away, if you have happen to be independently wealthy."
http://www.hardocp.com/article/2010/11/14/geforce_gtx_580_vs_radeon_hd_5970_2gb_performance