I have a vbulletin forum with around 200 members and 300 threads with 700 posts. Right now i have hosted the site on a shared plan at hostmonster with the unlimited features. So what is the right time to switch to a dedicated server?
Whenever you having 30+ users online at once on average. There really is no rule to this though. I would just stay on shared until your host recommends an upgrade.
I had a forum which was gaining 1,000+ posts a day and having 50+ members on at once, we didn't move to a dedicated server. Most shared servers are fine, but be sure to check with your host.
That is possible but your site would need to be pretty big for that. Some bigger sites have 1 server for all the files and one server for the database. We are still on a shared hosting plan and have no problems at the moment. There is no point paying for a dedicated server unless you need to.