The following tests were conducted to see how much difference a CPU really makes in gaming.
We started the test using a i5-4440 and then upgraded the same system to a i5-4670 and then the i7-4770. We ran 3 different benchmarks 3 times each and took the average score. The benchmark programs we used were Unigine Valley at Extreme HD, 3DMark 11 at Performance and 3DMarkVantage, also at Performance.
While the below graphs appear to show a significant difference, it really is only quite small. The difference from the i5-4440 to the i7-4770 is only about 3% on average.
This test illustrates that it is best to spend your money on the best graphics card before you spend a lot on the CPU.
If you are after a computer which is primarily for gaming, then upgrade the graphics card first and foremost. If you need the computer for other things than just gaming, in particular CPU intensive programs, then a faster CPU would be better for your needs.
We do however recommend at least a Quad Core CPU to get the most out of your graphics card & system in general.
We only show the difference with Intel CPU's in this example. AMD CPU's are very different, generally you will need the fastest AMD CPU to get the best out of most graphics cards. You can in some instances expect a 20% drop in graphics card speeds with an AMD CPU.