I was looking at the same question. I have an Asus A7V8X-X MB. Decided after research to go for Radeon 9800 Pro 128mb 256 bit. Gets top ratings etc.
Normally £125+ depending upon brand. However.....
I am having problems with conflict with by MB Via chipset. If you do a Google on this and go into different forums you will see that there is an issue. I have gone through all steps and patches but still getting BSOD in games. Have replaced card with old Geforce3 Ti500 and no crashing.
9800 will give go the best for your £££s but watch out?