Love the game.. one of the best RPG game I've played. I haven't played online very much, but most of the modules put online require the current add ons.. ie.. Shadows of Undrentide, and Hordes of the Underdark. When you go to play online, it will tell you which add on you need. Most required the most current (HofUnderdark)
Played ALOT online, it's good if you find a good server. All the servers are run by other owners of the game, no official servers. A lot of the servers are just friends getting together to play the games origonal adventure or other home grown mods.
There are also some persitant worlds out there, some are mediocre but some are exceptionally profesional and very large, just need to find one that suits your style...