I have a friend wanting to overclock my Intel E6400 processor. I heard they clock well around 55% is that true?

The ratios of overclock are dependant on a few factors (ie will it freeze up)
Anyway, I don't like overclocking CPU's; it causes more wear on the CPU and also runs hotter
It's at your risk, not your friend's

Trial and test

I have a notoriously bad attitude.....but.....I would let my friend overclock HIS E6400 FIRST and then get back to me with the results. Friends don't let friends drive drunk...pretty much the same principle. Don't let yourself be influenced by being called "chicken". That line's way too old to fall for.

well if you have a stock cooler, you can still over clock. Just remember the more you increase the voltages, the higher your temperatures will go. So if you plan on over clocking keep an eye on your temperatures. the following programs are very useful when overclocking:
1. your bios, don't over clock except through your bios
2. cpu-z will tell you what your current specs are
3. coretemp 0.95 will tell you the temperatures of your cores, and will tell you your Tjunction.
4. speedfan 2.33 will tell you temps, voltages
5. Everest will tell temperatures, voltages (compare with coretemp, speedfan)
6. orthos- run for 12hrs or more for stability. your core temps should not exceed 65c under load with small fft's