Just for fun, I appended this to my /etc/modules.conf file:
options nvidia NVreg_EnableAGPSBA=1 NVreg_EnableAGPFW=1 NVreg_ReqAGPRATE=4
The NVreg_ReqAGPRATE doesn't seem to work with agpgart, but that's to be expected since agpgart sets the rate. You might want to try that with a rate of 8 instead of 4 for your motherboard and with the NvAGP driver.
However, modprobe loads AGPSBA and AGPFW for me whenever I load X now. Sweet