If the client machines have X11 installed in some way, you can simply export the GUI of applications you start with the use of X11 port forwarding from X11. To do this, simply start your ssh connection with the -X parameter: ssh -X user@host. But this is usually limited to single applications and doesn't show a full desktop environment.
If you want a full GUI/Desktop environment, the classical (but not very secure) variant is to enable XDMCP capabilites on the server (for xdm,gdm or kdm). If you have X11 workstations, you could then configure the local login managers on the machine to use XDMCP to connect to the server so it's acting like an X terminal and you are using the local machine only to display the GUI. This is also possible in a window with the help of nesting X servers like Xnest or some X11 software for i.e. Windows.
If you want a full GUI over a slow link (something X11 normally sucks at, especially with todays fancy GUIs), you can use NX.