It's really very very hard to say.
Firstly, WHT doesn't just use a "hosting plan" but a cluster of servers, i think 4 in total though there's a couple of other boards on there also.
The resources you need would depend on alot of things, what type of software you're going to run, whether you're just going to run an "out of the box" copy or something with aload of mods and hacks adding extra queries.
I don't think anybody could say you'll use x amount the best thing is just to ensure that you are hosted somewhere that will allow you to easily upgrade if it's needed.
If this is a new forum though which it sounds like, not wanting to burst your bubble but i wouldn't jump in for a huge account or especially a server from day one, everybody always "expects" their forum to get hundreds of posts a day and be huge, but probally less than 1% of them ever do.
If you start small you can always upgrade, but if you throw aload of money away at the start and find it wasn't needed, you can't ask for that money back.