What is your Vcore set to? Mine was set to auto and you can keep it that way, but it might be giving it too much voltage and it will just be wasted energy and more heat would be output by your CPU.
If you have a decent aftermarket heatsink, then you could get away without touching the Vcore and just the clock speed, just make sure to benchmark it with Intel Burn in Test(about 10-20 minutes) and Prime95(run for 4-8 hours) to make sure it is stable, if you encounter any blue screens or freezes/random reboots, then you have to up the voltage if you set it manually or it's at stock voltage.
Those are about the range I saw with my 2600k which is pretty much the same as your 2500k aside from being 100mhz faster stock and hyperthreading.
For wanting to stick to auto voltage, I'd be careful as the voltage ramps with the VID which at higher clocks can get pretty nuts. I hit 1.3v at 4Ghz with auto voltage on my chip. If you're wanting to keep stock voltage like operation (drops when idle), then you need to learn about Voltage Offset and how to overclock with it.
For an example.. here's my setup..
I am currently sitting at a 44x multiplier with Voltage Offset set to -0.040v. This keeps me below 1.3v at full load and I still drop to 0.9v at an ilde. Each chip is different so don't think your chip will work like this right off rip. You need to learn about offset voltages before you even think about using it.
As for learning where your chip will go... Set a fixed voltage of 1.150v and start jumping up slowly and testing stability. The 620 will get you a decent clock, but either way keep an eye on those temperatures. I'd imagine you could get 4.4Ghz like I have without too much of a heat issue at 1.250v or so on fixed voltage.
The processor is designed to run a variable voltage, so changing it shouldn't hurt a thing. It's when you start over volting the processor that it really can become a concern. I consider over volt with the Sandy Bridge 2500k/2600k/2700k to be around 1.3v but Intel hasn't ever released an actual "normal" max on voltage. However max voltage they say is 1.5v which not many coolers can even think about keeping cool.
I try to stay below 1.3v, but some people have been running 24/7 for nearly two years at 1.4v+. There's no way to determine what voltage will cause faster or exponentially faster processor degradation. But for the most part, if you stay under 1.4, I don't see it reducing the CPU's life much.
yup what I was thinking aswell I will try 1.3volts and then just nudge up 1.31,1.32 etc however will think I will get more stable at 1.35 however it just depends on my chip after all they are all different.
First off Download CPU-Z - Realtemp -Prime95 - just like any overclocking you will need to enter the bios. Depending on your motherboard I always recommend going to the Save/Exit section and select restore to Defaults just in case some settings were changed if you were adjusting things so we can start fresh.
Main BIOS Screen:
One thing that I always recommend is disabling all the things you are not using such as eSata, USB 3.0 etc. After that depending on your motherboard you should have an Overclocking section so we need to go here next.
First go ahead and set your CPU Multiplier between 40 and 45. Since the Baseclock on most P67 motherboards should be 100mhz, 100mhz x 40=4ghz etc. This is the option that we will use because Sandy Bridge is very testy when it comes to adjusting the Baseclock.
Next go to VDroop and change this to without Vdroop. This will basically help reduce sagging while we run Prime95 or Linx and keep the voltage stable.
Next go to Internal PLL Voltage Override and set this to Enable- This is a very important step whenever you are using a multiplier of 40 and greater.
Next go to CPU VCore and set this to manual and then I would recommend starting at 1.300V for 4ghz and if you want 4.5 you will more than likely need about 1.325 but always remember to test in small increments in order to achieve a nice stable overclock. This may vary on your CPU whether or not you have the same identical one as someone else.
Next head over to the memory section of your BIOS and set the DIMM voltage to whatever the manufacturers specifications.
Now we need to configure some other very important features in our Bios which some are completely optional but I would highly recommend changing them for stability.
First go to EIST (Intel Speedstep Technology)and disable this feature. Basically this allows your CPU to throttle down below even the stock 3.4ghz when tasks are at a minimum so it is optional but I always disable it.
Next got to C1E Support and disable this feature. This is another power saving feature but enabling this might cause instability.
Next go to CPU C3 Support and set this to disable. I believe this is also called Sleep where the processor does not need to keep its cache coherent but maintains another state.
Next make sure you have Turbo Mode enabled. Most P67 -Z68 motherboards have this feature.
Next up, memory settings! Make sure to configure this per your memory specifications, or use the XMP function to use the built in SPD settings (if applicable)
Now you can go ahead and Save/Exit to Windows. I would definitely recommend having some Temperature software previsously installed on your system. I swear by Coretemp but there are many others out there. Make sure you keep a close eye on your temps after booting into windows.
Now that you are in windows check you clock settings with CPUZ and I recommend running a stability test such as Prime95. Another great one is LinX which is very intense. You can run it as long as you want and there are many different opinions as to how long you should run the tests to do some research online and make your decision.I usually run LinuX for two hours and then i run prime 95 for twelve hours so far those are what work best for me they might not for you.
That was very helpfull and if your wrote all that out just then, thank you very much my motherboard is a asus p8z68-v-lx i presume the settings would be the same for this board there has been little riots about using this board for overclocking if needed I can buy a new board but....