I don't have a Wii, but I can tell you that most
linksys routers now have UPNP to let things like
a wii configure the port usage automatically. 
UPNP is a big security hole, but alot of people
use it and love it.  I don't think your problem
is outbout ports rather than the wii needing an
inbound port.  I could be way off on this, but
just thought it may be of some help to you.  You
could probably go to the Nintendo site to find
what port you need opened.  

Good luck,
Ryan