In /proc/drivers/nvidia/agp/status. 3D performance was noticeably improved once this was corrected, so I'm assuming that AGP either wasn't working, or was running at a much lower rate.
Another odd thing in the 4191 drivers was that in /proc/drivers/nvidia/cards/0, my video BIOS showed up as ??.??.??.??.??
It shows up fine now though using 4191.
Did you compile MTRR into your kernel? (I'm just trying to remember all the little things that I did that APPEARED to fix the problem.)