Crossfire has micro-stutter issues and the 1GB of Video RAM is getting problematic now anyway for some games.
You are far better off a SINGLE graphics card and replacing your 6950.
Look around in a year or two and get an HD8000 or GTX700 series or whatever with 2GB or 3GB of VRAM that is at least 3x the performance of your 6950.
That is what I thought. I have always thought that a single card was better anyway. At least this time I will have a viable upgrade path unlike my AGP fiasco years ago.