NVIDIA Geforce GTX 275 Overclocked Performance Scaling
 

When it comes to overclocking, NVIDIA's GeForce GTX 275 is a capable part. Other people out there have gotten higher core speeds than we did but we achieved a very high shader clock. Our memory speed was also decent. And with this test the overclocked GTX 275 shows it can compete with an overclocked Radeon HD 4890: the GTX 275 never trailed NVIDIA's own GTX 285 while the Radeon HD 4890 fell behind in a couple instances.

At the same time, while the 4890 doesn't always lead the GTX 285, when the 4890 does lead it also typically leads the fully overclocked GTX 275 as well. These parts trade blows, and it is difficult to say that one is hands down better than the other. It really depends on the game and what you need out of the hardware.

http://www.anandtech.com/video/showdoc.aspx?i=3575&p=1