I recently got a Ge-Force 4, 64 MB graphics card for $25.00 US at
www.tigerdirect.com . I upgraded from a crappy integrated Intel Xtreme graphics. Check in your BIOS to see if you can disable your onboard video before you buy a GFX card. Also, make sure you are getting a PCI gfx card, bucause your motherboard probably doesn't have an AGP slot.
Hey, I have a motherboard with SiS integrated graphics too... It got fried by a cheap power supply, it smelled really bad. You should see the Cd-Rom drive, the chip inside melted like a volcano!