Nononono, you don't NEED to overclock it. The i5-2500 is great as it is. Only if you want to get more out of it, you overclock it. Just because you get the i5-2500k, doesn't mean you have to overclock it, only the option of doing so is there. If you don't know what you are doing with overclocking, don't do it. Yes, another person can overclock it for you, if you wish, but only the i5-2500k, not the i5-2500. Again, the i5-2500 and i5-2500k are exactly the same, except the fact that the i5-2500k has its multipliers unlocked, to overclock.
Slasher
Thanks for the info mate :)
I think I will get the i5-2500. Is enough for me.
For better performance, I need to change a graphic card and power supply isn't it?
Sorry, not going to go through this entire thread to look at what you have for a Graphics Card or power Supply...
I will say this, however...
If your Power supply is <500 Watts... and your card was working fine... Don't worry about it.
If you are thinking of getting a newer card (which MIGHT improve your FPS, but only really consider it if you NEED drastic improvement, like above 25 FPS) and the power supply is <500 watts, you should make sure your power supply is at least 500W or better, as newer cards are requiring a minimum power supply allowance.
And as Slash pointed out, you don't need to overclock it. He pointed out that the i5-2600k is unlocked so you can overclock if you wanted to. I only cautioned that if you are planning on it and you don't know how to do it, you should research first before trying. The i5-2600 is locked so you can't try and do that, but it should be fine for what you want to do in general.
Again, Overclocking isn't a must. It is generally for those who like to 'tweak' their system to get more out of it. How much more will vary depending on the user's wants, but the important thing as a person who has never done it, do not expect the returns to be 'leaps and bounds'.