vBulletin is a wonderful message board system (I've been using it for over a year now as an admin on various sites). I've never seen a PostNuke forum (I've seen post nuke with phpbb2 built in, but not a php nuke forum). phpBB2 IS a great board for free, but it (IMO) is nowhere near the standards of vBulletin. It's completely worth the money you spend on it.

Code modification is easy enough, in the admin control panel there's a special area for you to edit the code used in every template, as well as holding the original version in case you screw up.

I don't quite understand what you mean by "I can create the forum to do what topics I need", but if what you mean is have it like Sharky has it, with various areas for different subjects (programming, CPU's etc), then yes you can and it's very easy to do. If not, elaborate and i'll try and help. If you've got any other questions, feel free to shoot them at me and i'll do my best to answer them. *really likes vBulletin...if you couldn't tell *

~Scuttle