Unable to change display settings in Win

Trying to change resolution from 800×600 to 640×480 (to test for web devp). In Display Properties when I click Apply, I get an error saying: ‘The display control panel is unable to change the display settings. Please check your display hardware in device manager.’ Device mgr shows nothing wrong. I have the latest video card drivers. Monitor is KDS Visual Sensation VS-7i amd video card is SiS 6326 (AGP 4MB).