I was always told to work out the bandwidth you should work it out at:
500mb per user ?????

It will be 500MB per user if every user downloads 500MB worth of data from your website.
Otherwise... it will be some other number.
500MB "per user" doesn't mean anything - what time frame is that supposed to measure? Per day, month, year? And what's a "user"? A regular who reads and replies to every topic on your forum twice a day, or a registered member who registered once but never visits?
Bandwidth is measured in months in forum hosting.

- Are you going to allow people to post pictures and other media in topics?
- Will users be allowed to have signatures that are 1000 x 1000 pixels in size?
From a host's point of view, to do any sort of significant amount of bandwidth (>20Gb) you need a lot of visitors, but it completely depends on your traffic pattern, media you allow visitors to upload/download, etc.It would be interesting if anyone knew some accurate figures for calculating bandwidth for a forum.
 
	