I would recommend port forwarding also, but choose a 'high' number port as Birdy suggested. Although it's almost the same, portforward.com have not updated their instructions yet, see Manual port forward instructions & sample image for LW 5 location equivalent of the initial instructions they give at their site. I'd have normally suggested doing this last, but doesn't matter. Just make sure you set up a static ip address BEFORE you set the port forward rule on your Linksys WRT54.

If the Linksys router's address is 192.168.1.1, then you may wish to make your static ip address for this particular computer as 192.168.1.3 or 192.168.1.5 for example. Just make sure your other computer does not have same ip address on the network, and cannot be same as router.

BTW some software firewalls require a port number for firewall rules excluding a particular program. Make sure you use the same port as the one you port forward. Setting up an exception for LimeWire in your software firewall (click in link) (yes only have one software firewall active. If you use windows firewall then turn off 3rd party one or vice versa.)