A common misconception is that you need an Xserve if you are going to host a high bandwidth site. (i.e., if you plan to serve up large video files or a photography archive.)
In actuality, just pushing bandwidth requires very little processing power. The most important thing is to have a good connection that can keep up with demand.
In our facility, we have some customers who push three and four terabytes (4000GB) per month. Their machines don’t come anywhere near capacity.
A Mac mini is a very good high bandwidth server.