I'll echo COL on that, stock heat sinks are usually just barely good enough to cool the chip at stock settings, you might be able to use better thermal grease on a mild overclock, thats what i'm doing on mine, but its only a 200 mhz overclock. if you wanna go bigger aftermarket cooler is the way to go.
you can risk it if you want but would you really wanna risk paper weighting a cpu over a good 30-40 dollar cpu cooler