I upgraded from a Duron 1000 w/128mg RAM to a Duron 1.4 w/380mg ram and everything seems to run full speed now. One thing that I didn't think about, you guys probably already have.....
I was using the integrated sound off the MB, well, that takes CPU cycles to run. I put a $7 pci soundcard on and I think that makes a big difference, it has it's own processor so your not taking away from the main CPU running the game. Seems obvious now, but I never really thought about it. I don't think the small upgrade in CPU made the difference, I think it was the extra 256 of ram and the pci Soundcard.....
Just my 2 cents....