From what I've noticed so far the GX2 can edge out the GTX 280 on the lower end in about half of the games, the GTX 280 looks to win the other half. The biggest difference is that any test I've seen that uses AA / AF is when the GTX 280 is on top. Other than COD4 which seems kind of odd performance wise, once you add AA on higher resolutions the GTX 280 comes out on top.
EDIT: FS seems to not have an issue in COD4, and it runs better than a GX2. http://www.firingsquad.com/hardware/...ance/page9.asp