I've had similar issues before in older games. For example, bad textures or very slow particle/fog effects. For those I never really found a solution as it was just an incompatibility between the drivers and game engine. It is also possible it chose a shader pack for an NVidia FX 5700 rather than a generic ATI card. Does the game have a config or ini file anywhere inside the program files folder? And just to confirm, you remember playing this game on other cards without this issue, correct?
I didn't see either of those two files, EXT64.