Have you tried a snapshot? perhaps you'll have a little more luck with this card.. if not, you can always send a message to bugs@ directly, or via
sendbug(1).
http://www.openbsd.org/report.html
http://www.openbsd.org/mail.html
A full dmesg, and the output of
pcidump(8) with full verbosity settings might help, but I'm not sure.
I've also been in situations where the log file has been empty after an early lockup of X, you might be able to get at least some output by connecting to the system via SSH and following the /var/log/Xorg.0.log file using
tail(1) (..now you should have a partial of it in your terminal back buffer, which you can send to the developers).
It may not actually be locked up either, you could be sitting at a
ddb> prompt.. awaiting further commands.. but the video is left uninitialized.
Do you have a swap partition configured? around 2*RAM size? enough room in /var for a memory dump? then trying pressing enter/return a few times and issuing the
boot dump command (..blindly).
Give it awhile and you may see some disk activity, and eventually a reboot.. maybe
savecore(8) will get you a kernel core dump, so you can use
gdb(1) to get a
trace and help the developers find the problem, more info in
crash(8).
Good luck, keep us informed.