I had some trouble with a video card not being detected, so I changed that value. I discussed it on the forums a while back, so if you search on it it might help you. There is a sample sysctl.conf file in the /etc/examples folder. If you look up machdep.allowaperture in that file there is a comment to read the man entry for xf86.
I wonder if the problem could be anything to do with this?
http://daemonforums.org/showthread.php?t=10811