How long is a piece of string?
Your bandwidth requirements will potentially vary enormously according to a lot of things.
You can make some basic assumptions and do a few simple calculations though on the back of an envelope.
Using your own numbers, if you assume the page weight of a vanilla punbb viewtopic.php page (eg this topic itself) using the standard Oxygen style, with say 10 posts per topic, you are looking at a page weight of only about 30k per page view, including style sheets. (If your server has gzip compression turned on though on its pages that is only about 20k per page or MUCH less). So (very simply) 50 users browsing that same page would potentially pull down 50x30k = 1.5MB.
Using your hypothetical example, if 50 people posted a new post 10 times a minute for 1 hour (=50x(600x24) or 720,000 posts PER DAY = very high traffic!!!!!!) you are talking 50x[(10x30k)x60]=900MB/ hour or 21GB/ day simple bandwidth.
However if you meant 10 posts a minute TOTAL (ie 600 posts /hour ie 14400 posts /day) that would represent 14400x30k = 400MB /day. That's still quite a busy forum (busier probably than any punbb forum in the world), but not a lot of bandwidth - 12GB/ month or so.
But as I said - how long is a piece of string. If your forum has a graphically rich style with a huge header graphic and all your users have 20kb avatars enabled and you have lots of ads pulled off your own server per page view and a chatbox going and god knows what other mods running, a typical page view may represent 150kb. So you should multiply the above numbers by 5. If you host images on your forum via a gallery plugin or something, typical page views may represent 1MB or more of content. So you could need to multiply the above numbers by 30...
But complicating the calculation is the impact of server caching and compression. At various points along the way the pages generated by punBB will probably be compressed (by a PHP opcode cache or by Apache) and/or cached in one way or another. These usually will work to significantly reduce the actual bandwidth needed to serve x amount of punBB page views.
So any host that offers you 10GB a month bandwidth will probably give you lots of room.
In terms of finding a good host, just lurk around http://www.webhostingtalk.com - like every one else does , including mosts hosts - and keep an eye on the user reviews and also the specials various hosts offer from time to time in the 'Web Hosting Offers' forums. Also choose a host with a good online support/knowledge base and AN ACTIVE, PUBLIC USER FORUM. That can be a life-saver.
Oh, and you may find bandwidth to be the least of your worries if you anticipate running a busy forum... You will also need to start worrying about stuff like Apache and PHP and MySQL performance and optimization, and CPU load too. PHP and MySQL can be real performance pigs and will usually choke a busy forum long before it hits any bandwidth ceiling....