It should do 3ghz without breaking a sweat, and im sure there will be more than that even on stock cooling. Just monitor your temeratures, try not to exceed 70 under full load in Prime. As for OC ability of the chip, its one of the most "overclockable" ones Ive seen lately, especially considering the price.
70 deg ??? that is waaay to high. Try to keep it under 60-65 when running prime.
I bought E5200 with stock cooler and removed thermal compound that is preapplied on cooler and put a really fine, thin layer of AS5 paste on CPU.
I am running my E5200 on 3515MHz (10,5x335), 1,26V. Asus P5Q PRO motherboard. It reaches maximum of 58-60 deg when running prime, below 50 in games.
So my suggestion: lower the voltage bacause auto setting overestimates it and apply a thin layer of some good paste between cpu and cooler.
I didnt quite answer your question:
stock cooler is ok. you will not fix anything with a better cooler if you do not apply a thermal compound correctly. Preapplied paste on cooler is applied wrong: too thick, some cheap paste and it does not cover whole cpu.
I think that 4Ghz is doable on my configuration (cpu temp below 65 when running prime).