It completely depends how busy your forum is.
There are 3 factors to consider:
1) Database size
This will grow the more posts and users there are. Some forums delete posts older than a certain date to keep this in control.
100MB isn't a lot but is plenty to get going. Bear in mind the potential hassle and technical difficulty in moving to another provider in future if your needs grow.
2) Bandwidth
This will grow depending on how many visits your forum gets. If each "page" viewed consumes around 50KB (you don't need to count images hosted elsewhere) then 1000MB (1GB) of bandwidth per month will handle 20,000 page views per month. That's around 650 per day.
3) Processing power
The scripts that power a forum (ie pull the posts out of the database and display them etc) put quite a strain on a server. This means that your website will use more of the servers resources than the other websites sharing the same server which means the hosting company may force you to upgrade to a more expensive package on a less loaded server or instead they may throttle your forum so that it loads slower and if there is more traffic to your site than the throttled forum can deal with pages won't even load, they'll time out.
A busy site will eventually need its own server which can be expensive (ie £100 per month as an example) and really busy forums can require several servers to run them which can easily cost £1,000 per month.
Obviously not something you need to worry about now but it is worth bearing in mind for the future if your site does take off.
For now space with GoDaddy (or whoever) would be fine.
