hmmm... it's possible, would require an extended if/then statement in ubb_forum.cgi. You'll need the forum numbers for all your forums I think
Is there a reason you wouldn't want them all to display? When you upgrade to 6.4, you'll be able to take advantage of the ubb accelerator and the pagination that's been done in the versions since you last upgraded. Your server load will drop a bit with just that one change

We're doing that here (displaying all topics regardless of the user's choice) for that reason
