Normally, if you can connect with your client to your internet adress and port (5212), then others can too. If you want to be 100% sure, give your ip to poeple you want to join and let them try. With dailup this shouldn't be a problem. (exept the horrible "speed" you'll get with it)
If you want to be listed in gamespy, don't forget the "GameSpy Enable" in the nwnplayer.ini, it should be set to 1.
-Mel