Could you post the output of "lsmod"? What I'm looking for is agpgart. In most cases it doesn't play well with the nvidia agp driver. If it's not listed it could still be compiled into the kernel. Also is your SiS chipset one of the following:
o SiS 630
o SiS 633
o SiS 635
o SiS 645
o SiS 730
o SiS 733
o SiS 735
o SiS 745
It's taken from
Nvidia's page.
