Actually, I don't see why a terminal equipped with an RDP client can't connect to a Windows XP Professional PC. It's only RDP, after all - exactly the same as the server versions of Windows use.
Of course, it would only be one terminal. To connect a network of terminals, you need Windows Terminal Server. This could be NT4TSE, Windows 2000 or Windows 2003.
You would need any of the above if you wanted to use Citrix MetaFrame in any case - and the terminal would have to have an ICA client as well as an RDP client.
There is a solution to this - and you may well find it in the Windows 2000/2003 forums, or the Citrix forum48
Hope this helps