I noticed a serious hit to graphics performance, using ATI Radeon 64 DDR, after upgrading to XP Pro on one of my machines. But yet it didn't phase my other system, which was duplicate, except that the problem child had 512mb RAM, the other had 128.
After some experimenting I found that upping the AGP aprature size to 128 did the trick. For those who don't know this setting is in the bios under "Advanced Chipset Features" usually. Access the bios by hitting DEL or F2, depends on your system, during startup.
Usually this setting doesn't matter a whole lot because todays cards manage their own app size along with the OS (XP slacking), and usually have a min of 32mb on board memory, so settings above 64mb are usually not needed. A rule of thumb is to set the size at half of the total system memory.