have you checked the compatibility list at
www.xfree86.org with your current version of X? also, not familiar with onboard video, but there might be a way to disable it in the bios so the system doesn't try to use it instead... but again.. i might be wrong on that, never dealt with onboard video.
you can also try using SuperProbe to probe for you video card, or try not the Xconfigurator utility but try xf86config untility to try and configure your card, but first check the compatibility list at xfree86.org.... you might need a new driver or version of X
