Well, it seems that Vista doesn't cope with OCing as well as XP. At 3.9 GHz, I got BSODs on trying to log in. 3.8 GHz still got me a 5.8 rating...
With XP, I can at least log in and do a suicide run with my fans blasting.
I'm thinking you might need a quad core to get 5.9. 3.8 GHz is crazy fast, and if that doesn't get a 5.9, I don't know what will.