I have to admit that I've totally fallen for my 'quaddie'... It runs beautiful at 3.6GHz with just a 0.050 increase on the CPU voltage (1.36 Default/1.42 overclocked) . It would run at 3.8 (on air) but would be very warm and I'd also have to seriously ramp up the voltage. Anyway, benchmarking has proved that the 'sweet spot' is 3.4-3.6GHz with little or no gains above that..
I bet you'll be good for at least 3.6GHz and the awesome thing is, it only takes a quick change of the multiplier to achieve the overclock... What cooler have you got it running under?