My configuration: Intel quad core 4x 2,66 Ghz, ATI RADEON 4870x2.
Changing the .exe file, renaming it with "Dirt2.exe", as suggested here in another post, with the AAA filter disabled, it works a lot and solves many problems of glitching, and crash.
At 1860x1050 it runs at medium level details, 60 FPS.
At 1440x900 it runs at high level details, 50/60 FPS.
Without doing this, it slows much, with the same configuration it runs at 25/30 FPS.
In any case the framerate is hardly stable.
Hope this may help others players.

Sounds very weird. How can it be that naming one game to another game, which are totally different.. I get ini-tweaks and such but this. If that works. It's witchery. Still, until I get the game, I cannot try this.

ATI has different profiles for different games, meaning the settings are optimized for the game's engine, it just happens that Dirt 2 and Witcher 2 has a lot of the same settings that need tweaking to optimize it .

It has to do with CAP game profiles for certain games and other optimizations which are included in the drivers. As AMD already stated they are optimizing the driver for Witcher2 right now so renaming soon won't be needed anymore (or shouldn't).

Maybe if someone can give a more technical explanation that would help as well to understand.

the dirt2 profile helps a lot in cinematic scenes but not so much during actual gameplay.

For example with 2 5850 cinematics are pretty much smooth (60fps) with everything up except motion blur / ubersampling (motion blur is only used during actual gameplay not cinematics)

When you are able to control Geralt in the tent it's 70fps or so, but then talking to Triss goes to 30s. Going outside the tent also brings 37-45 fps max.

During Gameplay the fps are nearly the same as using only one Graphic Card at least for the minimum fps.

I supose the ATI hotfix drivers (and CAP files i guess) which are gonna be released this week (acording to the tweet they should be up today or tomorrow or so) will improve stability with better minimum fps and less ups and downs.