I got the game running VERY Well...
1024*768
Mostly high and medium details
around 35 to 50 fps
On
AthlonXP2400
1 gig ram
Geforce4 TI4200
AND THIS IS WITHOUT ANY TWEAKS
The difference is
You have a videocard that has better DirectX support... besides that, the performance on both is about the same (I have owned both cards in the past)
You're CPU might have a slightly smaller Clock speed... BUT
the 2400 @ 2.0Ghz has 256L2 Cash and runs on a 266Mhz Bus
The 2500 @ 1.9SomethingGhz has 512L2 Cash and runs on a 333Mhz Bus
So...
I think you're computer should run this better then 20 fps... SPECIALLY if you say there is no difference between low or high detail settings.
Also... what Chipset are you using ?
(KT400 and Nforce2 are the kings for the barton core)
My suggestions
Putting back all of the game settings on Default...
How old is you're XP Installation ? is it bugged out ?
Be sure not to have a Mass of programs running in the background...
I am certain that there is something causing you to lose some FPS... you're system should run the game on decent settings better then 20fps
Rad has a good suggestion ... If you do have quality ddr400 ram, mobo and PS (AND FAN)
The 2500+ Barton is a pretty decent overclocking CPU...
My personal pc has a 3000+ with good mobo and PS that I would overclock... but my ram is cheepness ddr400, ddr333