Do Quake 3 and Linksys PAP2 work without DIR-601 (when you just connect your host to the modem) ?
Based on the web search I see that UDP and TCP Ports 5060,5061 or 5723 need to be transparent for Linksys PAP2. Maybe you need to read documentation on Linksys PAP2 that came with it, port requirements must be specified there.
And for Quake 3 you need TCP and UDP ports: 27950, 27952, 27960, 27965 (http://portforward.com/cports.htm).
Jerry