Separate the database server and the web server in your considerations, because it's about how many web pages you need to serve, and how many database hits you need to do to generate and serve those pages and manage forum content.
So it could could be you only need a small web server, but a bigger database server for example. That might be the case where you have a smaller community who's extremely active in posting.
Really since it's hard to know the correct answer, but with what I've said above in mind, you're better off choosing a hosting service that allows you to change up and down your server sizes. Start small, see how the performance is. If it is fine, don't change anything. If you see your database showing some load issues, up your database machine. Maybe it needs more memory, maybe it needs more CPU, maybe both. If your web server is showing problems, up it.
You won't know for sure until you try it.