I would stay with v5. v4 is still viable as well as v3 but they will eventually be obsolete since the core product isn't under any further development. I reverted my site to v4 just to see the reaction from my users and it was actually pretty funny. They wanted the "old forum" back. VB5 isn't perfect and is lacking in the advanced features dept, but overall is a solid "modern" foundation to build the community on.

Hopefully within the next month or two, my house remodel will be finished and I will have some quality coding time.