For gdm, edit /etc/gdm/gdm.conf (on some systems this may be located at /etc/X11/gdm/gdm.conf). For kdm, edit /etc/kde3/kdm/kdmrc. In both cases, you must change the line in the [Xdmcp] section so it reads
On the client side, just type
Xnest -query remotemachine.com -geometry 1280x1024 :1
If you get a security error, try xhost remotemachine.com on your local machine (this adds the machine to the allowed list) and then the Xhost line again. If you get a "Server is already active for display" error, check that you remembered to put :1 on the end: Otherwise the server will default to :0, which conflicts with your existing desktop. You can also try :2 if there's a conflict with
For a more complicated setup, something like VNC may be better, but this method is quick, easy, and does a good basic job.