A few basics.
1. When you work on Terminal Server only keyboard/mouse clicks and the changes in the screen image are transferred across the network. A remote user could logon to a TS across the T1 WAN Link and consume as little as 33.6Kbps bandwidth, regardless of how big the excel spreadsheet is. TS uses Microsoft's RDP Protocol to transfer & display the screen images and it's encrypted at up-to 128 bits.
2. You can deploy the entire Windows Desktop to remote users, or just "publish" single applications, i.e. Excel, Word, Client/Server Databases... TO publish specific applications you can go from simple where you only publish one application to a user (which you can do w/o 3rd party TS Add-ons) or you can get fancy and publish any number of applications to a specifice user (or group of users), however this is usually done with 3rd party TS Add-on packages like Citrix MetaFrame, Canaveral iQ, Applauncher or WTSPortal. For small (less than 250 users) deployments it's usually sufficient to go with TS by itself.
3. Terminal Server Sessions can be presented as a second desktop/application on an end-users workstation or you can deploy thin client computers that run everything from the terminal server.
4. You'd most likely be adding another server, NOT changing one you have now. Since Terminal Server is a service that allows multiple users (sometimes 50+) to run interactive sessions (Windows Explorer Shell or other applications), you need a dedicated machine for this. It's best to deploy TS on a Member Server or Stand-Alone Server (if no other AD Infrastructure exists), so if you have a current Domain Controller, you add a Windows Server 2003 Terminal Server as a member of this domain.
5. Over your T1 Line you could run between 20-45 concurrent Terminal Server Sessions, depending on what settings you enable/disable and what other traffic exists in the pipe.
6. The ideal server for hosting Terminal Services is usually a dual-CPU machine with 1-4GB RAM & a RAID Controller. Compaq/HP DL360's are my normal recommendation. Windows Server 2003 has quite a few TS related improvements over 2003, i.e. display resolution greater than 256 colors (i.e. 16/24 bit), sound, greater scalability, user or device TSCALs (you'll need one for each connection)...
7.Best not to rush into deploying a TS until you've tested using it in a lab setup, which you can run on a workstation class machine. It's not extremely complicated, but involves more than just putting in the CD.
Post back with whatever questions you have.
Patrick Rouse
Microsoft MVP - Terminal Server
http://www.patrickrouse.com/ts.htm