Spliting Workload for Server

My company is going to buy a new server to split the load of our current file/DC/DNS/DHCP/Exchange/Print server.
Currenlty we have an IBM server with a 3.06GHz Xeon, 2GB of RAM and 650GB RAID 5.
We are thinking on getting a similar or better(dual/quad core) server and about same RAM, so my questions were…how should we divided the workload between these 2 servers? Should the new server host the exchange server or the file server? I have read that Active Directory should not be on the same server as exchange, but could we make the exchange server a backup AD?

there will be 15 users accessing both email and file servers.

Thanks