For a RAM recommenation, 100 - 200MB is usually the recommended value per user. It is also recommended that you run Terminal Services on a 64-bit server with the x64 version of Server 2003; this enables you to address more than 4GB of RAM (which is the limitation on a 32-bit system). By addressing more than 4GB, you can support many more users and you don't get the famed issue with the kernal RAM space filling up too quickly.
Obviously the RAM recommendation will change based on applications users are running. If they are doing intensive graphics and video activity in Photoshop or Flash, for example, then they would need much more RAM than using the web, Outlook and Word.
-tigermatt
