Quote Originally Posted by SKYMTL View Post
Actually, the HD 5870 is ~ 30% slower than the GTX 480 on average at 1920 x 1200 / 4xAA. Meanwhile, the HD 6870 is usually ~15-20% slower (average again) than the HD 5870.

The needed 40-50% improvement over the HD 6870 to hit GTX 480 territory is a huge jump no matter which way you look at it.
5870 1GB is approx ~7% slower in 25x16 resolution and ~8% in 19x12.
AMD needs ~26/30% improvement with 6970 at 19x12/25x16 resolutions to basically match 580GTX 1.5GB.
Also your numbers for 6870 are incorrect.

EDIT: I see Vipeax beat me to it .