Ideally you need to assess this for your own environment.
1) What applications will you be running?
2) How many users do you expect to be simultaneously using these applications?
3) Other aspects of the applications - external database connections, file/printer sharing etc
4) Printers - what, where and who.
Base server: PII/III Xeon preferred, but I'd recommend at least a dual 500Mhz.
128Mb RAM for the operating system, then 32Mb AT LEAST for each user. Running perfmon will give you a better idea of your per-user requirements.
A pair of mirrored smaller disks will serve you better than one huge disk.
Don't run Terminal Services on a PDC, Exchange server or other server that will give a significant performance hit.
Buy Todd W Mather's book
This will become your Citrix bible!
Good Luck!