Edit: I got it to 4.2Ghz, perfectly stable!! :)

I must say that the E8400 is one heck of a monster.

I've been long dreaming to have a processor at 4.0Ghz, lol... Especially a core 2 duo at that speed...

Here's the story (I'll try to keep it as short as possible).

Originally my desktop pc consisted of a core 2 duo E4300 1.8Ghz, 2GB RAM, 8800GT and X-Fi Xtreme Music on a vanilla P5B motherboard. Operating system XP Pro SP3. I had oc'd the processor to 3.0Ghz.

Now recently my dad wanted to buy a new desktop, which he uses only for internet and some office work. He went and got a quote amongst which were 4GB RAM pc-6400, Vista and an E7300 2.66Ghz core 2 duo. (o_o)

I thought heck the E7300 and the 4GB would be really wasted on my dad... they're so much better than my own E4300 and 2GB RAM!! So I managed to agree with him that he buys the parts just the same, but gives them to me and in return I give him my E4300 and 2GB RAM. xD He accepted, also that I get the Vista Home Premium instead and give him my XP Pro. So basically I was getting a totally free upgrade.

However when we were at the shop ordering parts, I asked them the price of the E8400 (which I know is a bomb of an overclocker) and to get it instead of the E7300 I just needed to add €40.

Naturally I opted for the E8400 and was lucky enough to receive an E0 revision as well!!!

As soon as I had everything set up, I inserted another 2x1GB in dual channel which I had lying around. They had had problems oc'ing with them on the E4300 for some reason or another (apart from getting only 2.9GB RAM recognised due to the XP being 32 bit). However this time I had no problem getting them accepted so now I had a processor running at stock 3.0Ghz on same P5B motherboard but with a quite whopping 6GB RAM.

The OverClock:--Yesterday my overclocking adventures started.

The E8400 had got a multiplier of 9 so at stock FSB it was 333. On my E4300 I had never managed to get FSB over 350 (always got the 'overclocking failed please enter setup' message).

So first I entered the bios, put the pci-e frequency at 100 and the pci at 33.33 because I don't want them oc'ing and possibly damaging sound and graphics cards...

I put the RAM settings on manual. Then I reduced the RAM from 800Mhz to 667Mhz since with what I was planning to do they'd be oc'd to more than 800Mhz if I put them at 667Mhz. (My RAM is running at 890Mhz right now if I remember correctly) On the other hand had I left them at their stock 800Mhz increasing the FSB would get them to over 1000Mhz and they'd probably fail me.

I also put the following voltages on manual and chose the lowest voltage available to me on the bios:

FSB termination - 1.2V
NB Vcore - 1.25V
SB Vcore - 1.50V
ICH Chipset - 1.057V

I put the RAM voltages at 1.9V.
I was suspecting that my different branded RAM will be creating problems (as they already did before).
My 4GB Apacer require 1.8V (as had done the previous 2GB Elixir I had) but the other 2GB Crucial Ballistix require 2.0V just to run at stock. Now with the overclocking all the RAM will be overclocked and I have to risk overvolting the Apacers just a bit in order not to have the Ballistix fail...

Now on to the really important stuff....

The E8400 needs a voltage (ie vcore) of between 0.85V and 1.3625V.

In cpu-z with processor at stock speed I was getting exactly one volt.

I entered the bios, set the FSB from 333 to 356 to get 3.2Ghz...
I put vcore in the bios at 1.2V.

Kaboom Vista loaded as if I had done nothing! I was like WOW normally everything hung up if I put the FSB anything above 350, now this not only POSTS but boots Vista and I'm able to run Prime95....
AND......... the temperatures are just awesome.... I mean at 3.2Ghz with both cores loaded at 100% I didn't get more than 40 degrees! Heck my previous E4300 used to idle at 40 degrees!

Then I got into the BIOS again, set FSB at 375 in order to get 3.4Ghz...
Amazingly, things went without a glitch....

I once more set FSB at 400 to reach 3.6Ghz... still perfect... I didn't do much extensive testing, just Orthos Prime for around 10 minutes both cores 100% small fft's test, but that was enough for me to try to get more...

Then I entered the bios once more, however this time I got a surprise The cpu vcore was set on 'AUTO' and I had no option to change it!!!

I thought okay weird... but wth... so I set the fsb to 423 to get 3.8Ghz.
This time I was disappointed, because my idle temperatures were 52 degrees (!!!) and just 12 seconds Orthos got me 71 and I just shut down the pc! Later on I checked cpu-z on the same 3.8Ghz setting and noticed in cpu-z (not in bios because that was an apparently unchangeable "auto") that my processor was getting a whopping 1.44V (OMG no wonder the high temps!!!)

To cut a long story short, I realised that, far-fetched as it might seem, the reason for my BIOS independently choosing to over-volt my processor was because I had been insisting on getting everything running using only 1.2Vcore... in BIOS settings.... which I know is terribly low.

However the reason why I used this method was that when I used to oc my E4300 I used to try to keep the voltage as low as possible until it wouldn't boot, then increase it until it boots, then try Linpack intel burn or Orthos Prime, and if it doesn't pass I increase it a tad till it passes.

HOWEVER....

Before installing the E8400... I had to flash the bios to the most recent V.2001 and apparently this changed some stuff.

Alas the previous bios had been safer in that if you don't give sufficient voltage (vcore) to the cpu you'd just get an "overclocking failed" message with the option to reset the bios again.

With the latest bios version, apparently the motherboard just feeds the vcore it deems appropriate for the processor to run at that speed... and it makes a bad choice because since for me it showed 1.44V in CPU-Z the "auto" it chose was the region of 1.5V which is too much for the 45nm E8400.

I discovered that the way to disable the bios to choose was to downclock again to an FSB where I get the choices for vcore visible, choose a more reasonable vcore (not the extremely low 1.2V), overclock again and voila': ---

Setting 1.3Vcore enabled me to reach 3.7Ghz!

To play safe and not have the bios overvolting the cpu against my wishes, I put the vcore at 1.35 (maximum for the E8400 is 1.3625V and that's the end vcore visible in cpu-z not the bios vcore) and I reached 3.8Ghz with no problems this time!

Then I just kept the same settings, put the FSB at 445 and got 4.0Ghz.
After that I reduced my bios vcore to 1.3V and rebooted with no problems at 4.0Ghz, ran Orthos Prime for 45 minutes.... played Fallout3 for 2+ hours... and before I went to sleep I put Orthos Prime running and it was still going strong 5 hours later at 7AM.

Conclusion: I'm currently running at 4.0Ghz.
Idle temperature is just 31 degrees. (Room temperature is 23 degrees).
Load temperature a very acceptable 51 degrees!
Just awesome for 4.0Ghz since my E4300 @ 3.0Ghz used to get 63-64 degrees at load.

I'm sure that either the E8400 is really really awesome, or perhaps for once I was really lucky and got a good chip... I've read online of guys getting the E8400 E0 stepping to even 4.5Ghz on air.......... wow....

To be honest I didn't strive for more. I plan to let 4.0Ghz kinda "sink".... for now, it's enough awesome for me.

I'm sure however that since it was soooo very easy for me to get 4.0Ghz on this chip (and I stopped, temporarily, voluntarily) I'm sure that I might be getting better overclocks when in the near future I decide to try to overclock this little beast to the max and see what benchmarking results I might get!