Hmm - that is a stumper. Everything seems to be in order, so why it refuses to do anyhting for you is a mystery.
My own setup is very similar, save that I'm only using a single graphics card, and it works fine.
Are you running antivirus on your system? I have McAffee Virusscan 2008 Plus, the latest, I think. Maybe I will be forced to remove McAffee completely, see if that resolves this.