Overclock, the main hardware is; Processor, memory and motherboard. Because these component will determine the limitation of overclock. For processor and motherboard, the limitation is the temp. High clock will increase. That will make the computer hang, or even failed to boot. But don’t worry about the temp. You can change the stock HSF the processor to other HSF which had better performance for reduce the temp.
Example :
The air cooling Scyhte infinity

Schyte Infinity offers awesome cooling performance and is a real alternative to watercooling. In the past when I was testing at a little high overclocks. I am convinced even first time users can do it. At this time Schyte Infinity was still the best air cooling who can make the cpu cooldown.

The Customs Water Cooling