The key issue is memory. And ofcourse how many traffic you are going to push through it makes it depend on the network interfaces and the speed of the cpu. Anything p100+128mb can push 10mbit with ease even tunneling it etc. And you can run HTTP/SMTP/FTP whatever also on the same box. But ofcourse it all depends on what volume you want and what exactly. The OS depends on everything but mostly personal taste. Pick what you want to learn, what you know how to manage... personal ;) Personally I use WinXP's for workstations and Debian GNU/Linux boxes for server type of boxes. Next to that a couple of other boxes running Net/Open/FreeBSD for testing and experimentation purposes.