Hi there.
I cannot recommend vBulletin 5.0 at all. We tried it for our forums for Valikorlia and it is just terrible! We upgraded from PHPBB to 5.0 and it was just a nightmare... We downgraded back to vBulletin 4.X and it is an amazing forum to work with, in my opinon.
http://www.kharlia.com/forumzzz - bad 5.0 forums
http://www.valikorlia.com/forum - good 4.x forums
Those are on the same host, same server for the database, but you will notice how much slower and unresponsive the vBulletin 5.1 Connect software is, I highly suggest you stay away from it.
IPBoard doesn't look too bad, but I am super happy with the last stable version of vBulletin 4.X - good luck in your decision!