I was able to reproduce the problem with the FreeBSD AGP GART driver, it should be resolved in future NVIDIA FreeBSD graphics driver releases. The attached patch should fix the problem in 1.0-8762; in order to apply the patch and build/install the updated driver, please follow the steps below:
# tar zxf /path/to/NVIDIA-FreeBSD-x86-1.0-8762.tar.gz
# cd NVIDIA-FreeBSD-x86-1.0-8762/
# patch -p0 < /path/to/NVIDIA-FreeBSD_kernel-1.0-8762-1586939.diff.txt
(edit src/nv-freebsd.h to enable AGPGART support)
# make clean install
I recommend using the AGP GART driver built into the NVIDIA kernel module (NvAGP), however.