HD 5970 is the best performing card today.Followed by GTX580.I don't belong to either camp as well,but this could be a criteria...Though,AMD having the strongest GPU on the market doesn't mean that they have better CPU's also.Not that their CPU's are bad or won't be enough for your needs,but as far as I know their best CPU is topped by average i7's...AMD CPU's are cheaper though,indeed.
Uh, the 5970 is a bit unfair, in that it's... Dual-GPU... A fair comparison is the 6970 to the 580...
Not to mention the 5970 costs $800-$1000, which is almost twice that of the GTX 580..
AND, if you look at the Anandtech review for the 6970, the 580 comes within a small percentage of the 5970, despite the 5970 having TWO GPUs, sometimes even beating the 5970.
Fastest single card? Technically.
Fastest single GPU? Disqualified, since it's Dual GPU.
Besides, it's not long until Nvidia comes out with the GTX 595, which is two 570/580s on one card.
Edit: You can http://www.anandtech.com/show/4061/amds-radeon-hd-6970-radeon-hd-6950/13. The Performance/$ is much much higher on the GTX 580. Comes within 10-20% of the 5970 and is nearly half the price.