The subjective opinion, playing Supreme Commander, Portal, Half-Life 2, Telengard, Dawn of War, CoD4 and UT3 is that these cards are damn fast. My informal game playing involves going into a game and turning everything up to the max, then playing for a bit. If it is playable, it is playable. If it starts stuttering, we back off a bit on a few things in a semi-scientific way until the low frame rates are gone.
With the 3870s, you would get some bogging here and there, not bad, but it did happen. None of it required backing off more than one notch on the settings tabs in any of the games. But in a week playing with the 48xx cards, we got none of that. Even with the most torturous in-game settings, these cards never let us down.
We could probably have gone into the ATI control panel and forced the issue, but with the in game settings, well, it just powered through things.