Hello, I'm Lolcat and I'll be assisting you today.
When you're second GPU arrives you are going to want to take a look at this guide: http://bitclockers.com/forums/index.php?topic=7.0
By default, Windows will not engage you're second GPU unless there is draw from a monitor. You can trick Windows and devise dummy plugs (as mentioned in the guide)
For your second question, here is a simple easy-to-use 'Profitability Calculator' you can use to determine "if it's worth it"http://bitcoinx.com/profit/
EDIT: Yes, 6950's are great
EDIT2: I'm not a mod I just like to talk like a sales representative.