Take a look at this page on Intel's web site, about your chipset:

http://support.intel.com/support/gra.../CS-010512.htm

It suggests checking the amount of memory allocated to your video card in the BIOS.

Take a look at this thread at KernelTrap.org:

http://kerneltrap.org/node/view/1896

It suggests that you need to use the "intel-agp" module rather than the normal "agpgart" module.