Mind pointing out where the 5970 4GB is 80% better than the GTX480?http://www.tweaktown.com/articles/3230/sapphire_radeon_hd_5970_toxic_4gb_video_card/index7.html
That's the graph showing high AA/AF performance, which is ideally where the extra RAM *should* shine.
FarCry 1920x1200 [8xAA]
GTX 480: Min FPS  Average FPS 
ATI 5970 4GB: Min FPS  Average FPS 
That gives the 5970 about 23% advantage in average FPS, and a 31% disadvantage in minimum FPS. And that thing will probably cost $150-$200 more than a GTX480.
*Incoming argument about how minimum FPS is more important, 89 v. 110 average is not noticable by the naked eye, etc.*