I have the same problem with a ATI1650pro AGP. The game starts fine but gets slower and slower within minutes till the point it is unplayable.
WinXP, Athlon3000+, 1.5GB Ram, 256 MB VRam, 1680x1050 32bit.
Switching to 16 bit didn't help, newest driver didn't helped either nor installing the Omega Driver package.
I use a 2 monitor setup. Turning the second monitor off made run the game faster at last. However this only a workaround, not really a solution.
At the moment I am running the demo version but I really would like to buy the full game. But it is a bit too costly for me to buy and discover afterwards that it become unplayable.
I wouldn't expect any help from ATI in this case, I doubt that they care about DirecX7 anymore.
I hope a solution can be found.
Cheers
Norbert