[Ntfu2]

Its hard to say really. As it depends on a number of things.

Heres how i always plan new/start up sites.

Plan their forums on shared hosting. Most of the time paying around $10 monthly will get you great hosting to get started. Its sad to say, but most forums dont ever have the need to leave the shared hosting environment. Reputable companies such as Hostwww or ASmallOrange will be good!

After you notice slow loading times, running out of space, using large amounts of bandwidth it'll be time to upgrade to a dedicated box. You can skip the "VPS" setups completely as they are useless because the price for them you can get your low end dedicated box. And everything actually decreases (bandwidth, storage, and the line speed into the box). The only real beneift is having a certain amount dedicated just to you.

After you exhaust your "starter" dedicated server, you can just keep upgrading it.

There is no need to start with some insanely large hosting package it'll just sit and go to waste. VPS systems really are about useless 80% of the time for forum owners, unless you need that level of root access for whatever reason.