The ports that will need to be open for x11 to work are:
Code:
x11 6000-6063/tcp # X Window System
x11 6000-6063/udp # X Window System
I've never had much luck with X11 over slow connections, I usually run VNCserver on the host machine and access the desktop session with a VNC client. On occasion I use cygwin for x11, since it's free and I like it better than Exceed, but even for local connections I find it easier to use vnc.