Recommended server setup for 20 clients

I'm setting up a network of about 20 clients with the following specs and requirements:

- Gigabit network (managed switch)

- Approx 20 iMacs, all NetBoot

- All users are network users with network home folders

- AFP file server for shared files

- Open Directory Replica

- DNS server

My questions:

- Can a single, new Mac Pro Server hande all of the above? If yes, what hardware setup should I order (CPU, RAM, Storage)?

- If not, should I split the load between several servers? Add a MacMini Server?

How would you set this up?

Your help is much appreciated. Thanks a lot!

Oliver