http://www.techreport.com/articles.x/16681
NVidia vs ATI and the upper mid range level - and Tech Report are one of the better technical review sites:
Conclusions
Jeez. We plow through all of those numbers, and I still can't declare any clear winner. What we do know is that, with the Radeon HD 4890 and its OC variant, AMD has succeeded in establishing a new standard for performance at the $249 price point. This is a most excellent development, and it has forced Nvidia's hand. Fortunately, Nvidia's response is quite respectable, as well. The GeForce GTX 275 is a worthy competitor to the 4890, and which card is a better value may depend on how prices shake out once both options are out in the wild. Remember, the 4890 OC card we tested costs $265, and if anything, it was an almost exact match for our stock-clocked GeForce GTX. Then again, the first GTX 275 has apparently popped up at Newegg as I write, and it's
priced at $259.99. So perhaps the 4890 OC is the most apt comparison—and the parity remains intact.