FP is on 3 gigs and 30 gigs bandwidth. I'm on the same hosting plan as them (Fowler suggested it). That should give you the picture of how much you need.
I have 3 forums and 1 website hosted on 1GB. If I were to cleanup the random files floating around, and 20+ extra themes that I don't use, I could fit all of that onto 500MB.
I suggest you get at least 500MB so that you have room for attachments and such. If you plan on a good size forum, 1GB is ideal.
Just a plain forum with no custom theme and such, you could fit on a 50MB plan if you needed to.
The only reason I have 3 GB space and 30 GB bandwidth is because it's the basic package on HawkHost - which are the best paid host I've been with so far. If there was smaller I'd get it.
I do store an awful amount of other crap on my hosting package though. I use it to test scripts, host random files and I even have my personal blog on it too. So far I've filled 800 MB.
Most hosts offer unmetered or high capped packages that have more than enough space for a forum. Just make sure the host has good speed and reliability is bearable. As stated in the thread already, 1 GB of hard disk space should give you peace of mind.