Like I said I will give you a good build shortly, when I get a second. Usually I am much quicker on these things, but I am swamped lately! Sorry!
Anywho, the processor affects the speed in which things run. Most programs now use 1-4 cores, with very few using more than that. The speed of the CPU is measured in GHz. Most come in stock at around 3.2GHz. With a K series CPU like the 3570k the multiplier is unlocked, allowing for you to overclock your CPU, or make it run faster than it is set to from the factory. With the 3570k you can go from the stock 3.4GHz to atleast 4.5GHz easily.
Now, with modern games however, more strain is put on the GPU than the CPU, making it slightly more important for gaming than the CPU.
The 3570k is a stout processor capable of running any game you need, which is why anything over that is overkill. To give a simplified example, lets say someone needs 10 pounds of food a month to live. The 3570k gives you 12 pounds a month, but the 3930k gives you 20 pounds per month. Why go with the 3930k and spend alot more if the 3570k suits your needs.
If you will be getting into video editing or the likes, then I do recommend an i7 processor, because they use hyper threading which gives them an edge over the i5's in highly CPU intensive programs.