The Story of David Kaspar

How should I configure the fw + port forwarding of WAG to use with emule ?

Create two rules in port forwarding, one for the TCP port, the other for the UDP (if you use Kademlia). (and check the box)
In Emule, you must have the same port numbers (no matter what number...).
Try to set a small number of sources in Max Sources/file and a "small" number in Max connextions. My settings are : 150/500, based upon my experience of another modem/router with small NAT entries. (Perhaps it is different with the WAG54G?).

You might want to try port RANGE forwarding.

It allows you to select both TCP/UDP and a range and I find it sometimes better than creating several single port forwards and doubling it if both UDP/TCP is needed.