I recently helped a friend get two XBOX 360s online behind his Linksys WRT54G v4 router and thought I would post how I did it, since there really isnt that much info out there online on how to get this to work.
Set XBOX 360 #1 to the static address of .102
Set XBOX 360 #2 to the static address of .103
Set the DMZ in the router for the .102 address of the first XBOX 360.
Forwarded the following ports on the router to .1 on both UDP and TCP (the router itself not the other XBOX 360 or it wont work):
His version of the router doesnt have an UPnP option and cant be updated to a hacked firmware to help out. But this worked with no problem. Now both 360's can voice chat and host and join any games.