aigomorla is right, while it is expected to get 3ghz on stock voltage, it is not guaranteed, only stock speed is guaranteed. Back in the day the Q6600 G0 were expected to go even higher, but their quality diminished since (I heard the theory that intels "alpha" plants ramped up to new 45nm process, so the lower end plants are now making the 65nm CPUs, makes sense).
Also the quality of components you use makes a huge difference.