I have a similar Mobo (ASUS A7N8X-X)
My video card is a Sapphire Radeon 9200SE 128Mb
My CPU is also an Athlon XP 2500
I'm running Fedora Core 1
So our systems are comparable.
I'll spend some time on this over the weekend if I can.
Hopefully one of us will find the answer soon. Good luck
BTW: if you run your system without installing the ATI drivers, what fps do you get in glxgears?
I get around 63fps full screen 1024*768
I also get around 16fps in chromium with full detail.
You should get a bit higher cause your card is better