YouTube Spotlight: Why Don't Multiple GPUs Scale Properly?
Check out this explanation by Linus on Techquickie as to why there are SLI and CF scaling issues. I am a fan personally of single card setups because not every game or application will use both video cards. I only recommend SLI/CF once you already own the most powerful single GPU and have no choice but to add another. Click the video to play it or the source link below.