How much will it affect

Hardware Hub CPU
1

i have a 3ghz cpu right now will it make a big difference if i upgrade to a 4ghz cpu

2

there are a bajillion things to factor in BESIDES freqency, like arch. L2/L3 cache size and speed, memory controller, how many cores and FPUs does it have and it goes one, what cpu do you have and what are you looking at and what for?

3

Speaking in broad terms, you will see a performance increase, might not be huge, but in all odds it'll be noticable.

4

What CPU do you have and which one would you buy?

5

i Have a pentium g860 i would upgrade to an i5 3570k and overclock 

6

Well then it would make a huge difference. What motherboard do you have though? Not all mobos that support pentiums support Ivy Bridge CPUs. Also, it's not recommended to overclock if your motherboard doesn't have much vrms or just sucky ones.

7

i would get a new z77 or something like that