I'm running a Core 2 Quad Q6600 at stock (need to reapply thermal paste to oc. it back at 3GHz). The difference is really amazing.
If you use Ati's monitor scaling (keeping aspect ratio or not) as opposed to your LCD's monitor scaling, you're in for a treat. Plants vs Zombies was previously blocky as hell in fullscreen but it looks nice and smooth on my 4890. What surprises me is that this effect falls onto other resolution locked games.
3D performance is through the roof. Lost Planet: Extreme Condition, at it's maximum settings, used to run decently at 1440x900 on my G92, however I'm running it at 1920x1080, maximum settings, 6x AA and 8x AF and it still runs faster than it did on my G92.
Many 3D games feel more performant. Sacred 2 runs better at maximum settings posible, 1920x1080, with 8x AA and 8x AF. Sacred 2 with shadows disabled didn't run as fast on my G92. The Last Remnant feels noticeably faster, but ran well on my G92.