My suggestion would be to abandon the onboard SiS graphics & use a dedicated graphics card (pcie)
This will bring 2 benefits....
1 - free up system memory currently being used by the onboard graphics. (this will improve XP's performance)
2 - i imagine the open GL performance would improve greatly (SiS aren't renown for there graphic chipsets in the same way ATi & Nvidia are)
possibly not the answer you were looking for, but maybe a less stressful way of resolving your problem.
PS i cant really recommend a graphics card, graphics aren't really my thing