InsideHW.com has compared ASUS GeForce ENGTX275 and ASUS Radeon EAH4890 graphics cards

With its HD 4890 model ATI raise a lot of dust and made huge impact on graphics card market. Despite the fact that this card should have been "just" optimized Radeon HD 4870, it was much, much more than that. RV790 is capable of withstanding frequencies that were unimaginable for "old" RV770, and just the fact that it can operate at 1GHz is completely different story. It is clear that this kind of overclocking freedom made this card a very interesting purchase for all enthusiasts.

ASUS GeForce ENGTX275 vs. ASUS Radeon EAH4890