The E6400 i run @ 450FSB x8 for 3600 @ about 1.5v in the Bios before vdroop.
The E6600 I swapped with a friend for a previous E6600 chip hoping to find a better clocker
I run it @ 378FSB x9 for 3400Mhz @ 1.5v in the Bios.
Both need LOADS more voltage to get more out of them after that, especially the E6600, more than im prepared to run through them on air tbh so I dont know if they will hit much higher.
I'm having a few problems with my DS3 Bios at the moment which isnt helping, on/off/on/off reboots and the like. I really would like to find 3.6Ghz on that E6600 then I'd be happy