as long as you can keep it cool, and supply the correct voltages, there is virtually no limit. Your motherboard has to scale with the proc too. (MCH, Memory, etc)
For example, my E6600 running on bad axe is @ 3.6ghz, stable. [ddr2 800]
Some have gotten the 6600 between 4.5-5ghz stable. Also, there is no universal limit to overclocking a processor. All of them are not identical, nor are the systems of those who overclock.
The stable limit that you attain is related to all of these factors.
Id get another fan though, definitely. Im guessing you were informed that 3.6ghz was the stable limit for the stock fan, due to thermal limits.