i can tell u going from a 3870 TOXIC to a 4870 i am getting solid improvements.
in 2142 i had about 5 settings set to medium, rest on high, no AA and my fps were fine until there was an artillery strike when it would stutter. i now play with everything on high, 4xaa and while my fps lower during arty strikes, they stay above 30. game looks noticeably better.
in world in conflict with the 3870 i also had about 5-6 settings on medium or checked off, rest on hgih, no AA on - id get terrible stuttering when a nuke dropped and a few other slowdowns but not much - mostly nuke time - i now run everything on high, 4xaa and 4xaf and even when a nuke drops i dip to 28fps for a moment then back up which is fast enough to not stutter at all. game looks noticeably more polished.