SBS cannot be used as a terminal server anyways.
A 2003 member server with the specs you noted will work fine on your network. How many users do you anticipate concurrently using the TS? Do you need 750GB of storage with the RAID you have quoted?
The second server you currently have, what roles does it serve? Depending on this, its hardware, and the user count it may be an option to use as a terminal server.
1GB of RAM, BTW, is pretty low for a SBS Premium server, another Gig of RAM will help performance depending on load.