Game lagging and slow performance of pc.Suggestion about a gaming rig?

Actually, I don't have so much money to buy the i7.
If considering the price, I should buy i5 2500k or the i7 2600k?
Which one is more worth it?

Well, I think you should go with the i5 2500k, its more than perfect for your needs, and if you need more power, you could simply over-clock it, as it has an unlocked multiplier. If the i5 2500k is too much powerful for your needs, you can buy an AMD, which is cheaper, but you will get less processing power.

SlasherIT said:

Well, I think you should go with the i5 2500k, its more than perfect for your needs, and if you need more power, you could simply over-clock it, as it has an unlocked multiplier. If the i5 2500k is too much powerful for your needs, you can buy an AMD, which is cheaper, but you will get less processing power.

Well, thanks for the suggestion but I don't know how to overclock it.
Overclocking need some skills, yes?

You won't need to overclock it, think of overclocking as a bonus.

SlasherIT said:

You won't need to overclock it, think of overclocking as a bonus.

If I change to the i5-2500K , is it okay with my old graphic card? (GeForce 9500GT)

That depends entirely on whether your motherboard supports it or not.

Of course, when I change to i5-2500k, I will also need to change new motherboard because I have an old one....
So is should be okay?

What motherboard exactly?

SlasherIT said:

What motherboard exactly?

I'm using Intel Sorento D945GNT
Quite old....

Yes, you will definitely need to buy a new motherboard to support your processor. Also, some other components in your existing system might not be compatible with your new motherboard.