I've been involved with online Forums for ten years now and in my humble experience I've found the less categories the better as a rule. And vbBulletin has a search engine which works reasonably well.
To have a dedicated 64 Bit category could encompass many Linux Distros as well as XP, Vista and W7 which would to my mind simply create confusion.
As Ian has mentioned, many topics in the software section of the forum relate to both 32 Bit & 64 Bit versions of the three Windows' OS's which to my mind is preferable to specifying a query as 64 or 32 Bit.
The software forums as they are work well, in my opinion, I see no reason to change them.
But thanks for the suggestion - food for thought - and welcome to PCR UK