Many thanks for the mods Xiaopang, especially the second one. Previously I could only get an average of around 20 fps (even on the lowest settings with your config mod), but since trying the init mod I can play the game at a comfortable 35fps average.
I've played the first two areas so far and haven't encountered any problems. For some reason I get the best performance on Shader 4.0, which is handy as it means I can play at 800x600 without black horizontal borders.
Anyway, thanks again. Hopefully one day I can play this at max settings with all the fancy effects turned on....
[Core 2 Duo @ 2.8 Ghz // 4GB RAM // ATI Radeon HD 4650]