I have a similar setup at my house - DSL service, WRT54G router, using EchoLink via WiFi. On my router, I go into the Applications and Gaming tab, then look for Port Triggering. In the Port Triggering screen, I define an entry for EchoLink:
Triggered Range: Start 5198, End 5200
Forwarded Range: Start 5198, End 5199
(and click the box in the Enable column)
Then Save Setting, and this way you don't have to specify which computer will run EchoLink. If the router sees something going out to any of the 3 ports EchoLink uses, it will forward inbound traffic back to that computer on your network. Normally, you'll make an outbound connection on port 5200 - a TCP connection, used to log into the EchoLink server(s), and get the current list of those online with EchoLink at that moment. Port 5200 is not used from EchoLink back to your PC, but 5198 and 5199 (UDP) are used when you are talking with another station.
Make sure you have up-to-date firmware on the router, as old versions did not have the Port Triggering option. Port Forwarding will work, but you will have to specify the IP address of the computer on your network that will run EchoLink. If you might use more than one computer with EchoLink at home, this can become annoying to change that IP all the time.
Good luck and 73!
Patrick WD9EWK/VA7EWK http://www.wd9ewk.net/