if i buy rainbow six off steam.
will it go to the steam server or the offical rainbow six server?

plus, if i buy it from steam, how will i get the cd key?

Do yourself a favor and don't waste the money on it. Buy Raven Shield instead. It's a much better, and less buggy game.

No. when you buy the rsv through steam, you will play on regular rsv server. There is no steam rsv server. And when you buy the game through steam, it will direct you to a certain page, and they will show the cd key, but not to sure about the cd key. But there is a another way to get the cd-key is when you have rsv installed u can go to My Document -> My Games -> Ubisoft -> RSV -> then there will be a file called profile, and it will display the cd key when you open them.