It depends on the size of your forum. For a small forum, you should probably keep it on shared hosting unless you need a VPS or dedicated hosting for other purposes. A VPS or dedicated hosting for small, low-traffic forums is excessive and would be a waste of money in my opinion. When your forum gets much larger and starts getting more traffic, a VPS or dedicated server will be necessary. Forum Promotion, for example, saw the need to move to a VPS around the time that we reached the one million post mark. Your need for a VPS or dedicated server, however, will be based more on traffic than on the size of your forum.