Am I to understand, that Steam can allow you to set up multiplayer games for ANY PC game?

I recently downloaded it for the free Portal Demo, but I noticed the option to add "non-steam game" so does this give the voip functionality to games like Baldur's Gate and NWN? 

No, not at all.

No, it needs specific Steam support to allow for chatting and such.

But...the new Community overlay should work for any videogame that you put on your Steam list, and through that, you can text chat or voice chat while using the overlay ingame...if I understand that correctly.

no i dont think it can work for all

 No, it's merely a nice feature of Steam and it's acting as a launcher for your games. Rather than have to search for the game launcher icon in folders/desktop/start menu/ect, you can set it so that you can launch them from Steam instead. 

Merely conviencence

No.

PS: Steam is crap. Seriously, it'll eat your brains.

^Steam is okay....it's certainly a lot better than games for Windows Live.

Okay, so with that ray of hope and dazzling light shattered before it could even come into blossom, does anyone know of any program that can function in this manner?

Xfire is a chat program that supports tons of different games...don't think it has voice chat though...and it is somewhat limited.  You're asking for a program that can allow you to join servers in random, non-related games...lots of games use different systems...some require you to login, etc.  Steam while playing Steam-supported games is the smoothest system out there...otherwise you're just going to have to set up multiplayer games within the game itself.  Not a huge deal in my opinion, but then I play Steam-supported games 80% of the time probably.

yeah, i was hoping for a good (free) program to give a little Voip support for Baldur's Gate and NWN