I have the same board and am having the same issue
I average about 9 frames a sec with any detail setting.
I have and Atholn XP 1.667 Ghz
1 gig o Ram
geforce 4 Ti 4600
ASUS A7N266-C mother board.
my Xfree86.0.log states that:
it failed to verify AGP usage
I took agpgart out of the kernel and am running nvagp 1 to no avail