Estimating bandwidth requirements for web/mail server

[John Almberg]

I have a FreeBSD web/mail server in a colocation facility. They offer  
many fixed and burstable bandwidth options. I am currently using  
512Kbits fixed, which limits data transfer to around 64K up and down,  
simultaneously. This works okay at the moment, but I'm wondering how  
this will hold up as I add websites to the server.

Other than empirically measuring load on the box, is there a way to  
predict or measure how much bandwidth I need? Any rules of thumb?

Thanks: John