Is it wise to lap a cpu (e8400) and how much temperature I can able to get it down to?
Thanks!!!

Why bother ... the chip runs like ice and overclocks to 3 plus gig without a voltage increase.
 
Find the sweet spot and lock it in and enjoy it.
 
Lapping a cpu is beyond enthusiast  ( I assume you mean taking the IHS off first and lapping the cpu die) ... your in a world on your own.
 
A very delicate world

 
4GHz is the w/Vcore boost. 3.6GHz is the no Vcore boost norm.
 
Lapping is normally just sanding the IHS flat and the HSF as well.
 
For the OP, usually it can make it roughly 2-3c cooler. But that depends on your luck.

Also depends on how concave the CPU actually is.
 
Some 65nm chips were pretty concave and saw better increases.  Others were already pretty flat.
 
 
Put a razor blade up to your chip to see how flat it is.  If you can see that it is pretty concave, you may be able to lower temps a few degrees.

when i lapped my q6600 it dropped a good 10C.

If you plan on lapping the CPU you should also plan on lapping the hearsink too. (depends on how good the heatsink is)

  Anything I should be aware of while lapping a cpu?   I have a HR-1 PLUS 6 HEATPIPE thermalright and I did lapped it to a mirror finish... Also using a ULTRA 90 for nb and sli HR-1 for sb (lapped also just to be flat)... All this are for DFI LP LT X38 MB....

You will void the warranty by lapping it.
Take the plastic cover off the bottom of the CPU, else you might get plastic over the contacts.

 
  I have E8400 3.0 overclock to 3.6 with no voltage increase but many people say it will be voided once it get overclock in any way and I have an OEM CPU how that affect my warranty?
 
  Is static will not be a problem if I lapped a cpu (planning to wear a latex gloves and cover the cpu connector with paper then tape)