I5 2320 vs i5 2500k

2500k allows you to over clock the CPU. at stock speeds with the GTX 580, you won't notice a real world difference between the i5-2320 and 2500.

i5 2320 - not a K model.
What price difference are you seeing between i5 2320 and i5 2500K?

Ummmm.... I am assuming you mean INTEL i5 2500k. As pointed out there is no such thing as an i5 2320k.

The "k" means that the i5 2500k has an unlocked multiplier that will allow you to overclock the CPU for better performance. BF3 and Skyrim are examples of games that loves high performance CPUs, the higher the performance, the more frame rates you will get. Since Diablo 3 is likely to be a CPU bound game (like StarCraft 2), a more powerful CPU will also give you better performance.

As a side note, for game where the combination of the i5-2500k and GTX 580 can give you more than 60FPS, your monitor will limit the FPS to 60 unless you have a 120Hz monitor which is capable of displaying up to 120FPS.

Thanks for your help i guess I'm kind of a noob.