Seems very strange
What does lsmod | grep -i nvidia gives you ? (as root)
It should say something like
nvidia-agp 7388 1
agpgart 31016 2 nvidia-agp
If it does not then something does not got started right, but since you have done everything right I am not sure.
Since you seem to be no newbie I guess I can asume that your kernel is 2.6 (ofcoz)
Earlier hatman (post #12 in this thread wrote)
I Dunno if it matters but i had agpgart compiled into the kernel instead of as modul (default) because i read about it in another thread.
He has much the same setup like you so maybe that will help.
I am very busy and might not be able to look futher at the problem the next couple of days. And I am not sure if I can help.
C++Boar