Single server vs. Multiple servers?

Our current situation is:

-50 users (LAN) medium to light users.
-1 single server used for apps, print sharing, and mail exchange.
-All filtering and monitoring is at the firewall rather than the server.
-Web-hosting is done by the ISP.

I’mconsidering having 10-15 users run in terminal server mode. (For admin purposes) I would think it would be wise to have a seperate server for my TS apps. A friend feels it can all be run on one single server. Any opinions or thoughts? Other thanthe obvious issue of overload, why else would it be good to have 2 servers?

Thanks