hmm, i'm out of ideas. can't see anything strange in xf86config and in the log.
1. try to see if the card works in graphics,
try the svgalib (there are demo/test programs in /usr/lib/svgalib/ )
2. try to set the safest and slowest options in bios, mainly about agp. You know make it run, then make it run faster. (there was module option for apg 4x and specific MotherBoard)