Download the latest SiS Chipset drivers from here:
http://www.sis.com/download/agreement.php?url=/download/
You'll want at least the SiS AGP(GART) Driver and SiS IDE/SATA/RAID Driver(s), you may also want the SiS LAN driver, assuming your network chip is an SiS one.
Once installed reboot.
Make sure nothing else is running in the background using memory while gaming - your problem sounds like a memory leak in an application, or in the video driver itself (which may indicate you've never installed SiS drivers and you're still using Microsoft 'generic chipset' compatible ones for both AGP/GART and IDE/SATA/RAID) - Thus my suggestion above.
XFX Support might be able to help you more with this, but not sure if they only support their products, or will delve into mainboard chipset / video chipset + drivers + configuration in general.