I'm having this same issue:
This is a serious problem &, from what I've been finding after many, many hours on the internet- it's more common than you think. It'll take a true computer tech/expert to solve.
There is some sort of glitch with VIA chipsets & certain video cards. It relegates a 4X AGP video card running on a 4X motherboard- to a 2X AGP board speed. Mine is one of them, unfortunately. I've tried nVidia (GF3 Ti 200) & now ATI AIW 8500 128mb. I have an L7VMM mobo, 1.4GHz Athlon, 512DDR, XP Pro- & I can barely break 8000 on Mad Onion. And yes, all of the current drivers (Windows Update, sound card, video card, chipset, mobo-controller/bridge, monitor) installed correctly & doublechecked through device manager.
So, the seeming consensus of opinion is that it's the VIA chipset not communicating correctly the actual AGP bus speed.
Recent fly in the ointment is the additional bits I've read talking about some mobo's being 4X capable but shipped physically jumpered to a default of 2X. Is this correct?
I would like to have my Radeon running to it's fullest potential- it's the whole purpose for buying a nice card & 4X capable mobo.
If someone could enlighten me as to how they may have gotten around this, I would be eternally greatful (& would spread the news on every group I could find).
Tracy