As far as speed is concerned as well, you never mentioned what type of server you have this on. I assume it is a dedicated server considering the size of your site. But for instance, are you running a RAID configuration? How much RAM? Dual or single CPUs? These things can make a tremendous difference in speed.
Also, like Astaran said, tweaking MySQL is much more important with threads/vb due to the nature of the software... for tweaking MySQL for busy forums, you may want to check out this thread (assuming you have the newest MySQL installed)
http://forum.rackshack.net/showthread.php?threadid=20983&highlight=my.cnfJust updating my server's my.cnf file with some settings mentioned in that thread, my server loads went down about 30% over a standard mySQL config... just more options to look at
