X-Win32 can connect to linux hosts even behind a firewall.
Open up TCP port 6000-6005 (plus X-Win32’s base display number) in both directions
Open up UDP port 177 to accept incomming connections from any port.
If you are using Gnome open up TCP ports 16001 and TCP 35091 in both directions.
If you are behind a router using NAT, map those ports to the IP address of your local computer on your LAN. The only other thing is make sure on the Network tab under the X-Config menu, that your display is the correct IP address. This is the address you are telling the host to send the session back to.
If you are using SuSE linux firewall, edit /etc/sysconfig/SuSEfirewall2 changing the default options to the following