"I get a system stop at the mounting blah blah"
What's the system mounting? The root filesystem, or other filesystems? Does this kernel that you compiled with USER_LDT and SYSVSHM (and AGP device support, I guess) enabled at least boot without the nVidia drivers?
I'm thinking it might be something like Mandrake 8's issues with reiserfs -- it didn't install an initrd, and it built reiserfs support as a module, so if you formatted / as reiserfs, there wasn't support in the kernel to mount it at boot time, and you'd get a "kernel panic: can't mount root device 03:05" on boot. But I don't know a ton about *BSD, so it may not be that -- indeed, that may not even be possible, I don't know.