"So we've established that ATI cards don't have the problem... I wonder why Nvidia cards do?"

I'm not convinced we've established this is a Nvidia problem... I've used just about every generation of Nvidia since the TNT, and I've not been able to reproduce your problem on a GeForce2,3, or 4. (Neither have I been able to reproduce it on my ATI cards either.)

I'm starting to think it might be some other problem... could it be because you are still using win2k? Just a shot in the dark...

EDIT: Mach... I think he's refering to a frame rate stutter he gets when the screen is full of translucent sprites. I believe what you are pointing out is sprite clipping with world geometry... that is normal and sadly an artifact of the technique.