Exactly what 6950 do you have? Not all of them unlock.
Correct about the motherboard, you need an Intel chipset/socket motherboard for an Intel CPU. You were wanting to buy a new mobo anyways, so I was just saying for when you need crossfire to get an Intel chipset mobo.
And about the case, have you seen a full tower before? They're HUGE. A mid tower case is more than enough space unless you're using over 6 hard drives.