Business Center minimums

Hello,

I want to build a small business center with 1-3 domains and a web server and file server and print server and all with back up.

How many computers do I need and what functions should each systems role be.

I have 4- IBM e-server rack mount 3.0 dual xeon systems all with 2gig memory and 3-36GB storage each.

What are some configuration sugestions do you recommend?

What small aditional equipment do I need.
I have 4 licensed server 2003 software.

Thank you for your input.