ive always installed over the top with ati drivers , but since 10.5 all i would get is blue screen on boot up and have to roll back . Today ive unistalled old driver first ( 10.4a ) Then popped 10.9s on and my 2x5770 are running great . So happy days 

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lol. So in my case the 10.9a hotfix driver was to blame, it's simple and that's the truth