Actually the stock fan is typically good enough to overclock fairly well. Assuming the chip itself is capable, 4.5GHZ with the stock HSF is not unheard of.
Maybe it is? Do the FX CPUs run cooler? I couldn't imagine they would considering how much power they suck. But theres no way you're getting past 3.7GHZ on the stock Phenom II cooler. My *** was throttling on that cooler under load before I OC'd.