By how much does it really increase the speed of your machine?

Is it a good idea
And

How do you do it?

It depends on the particular component and motherboard. See ExtremeTech Overclocking Guide - Why Overclock? Performance Comparisons for some benchmark numbers and discussion.

Depends on who you ask. I personally don't think so, as the gain often comes at the expense of stability. Additional cooling and power is often required as well. There is also a risk of component damage. For me, the time spent fiddling with rates and voltages would be better used working to gain the funds to buy faster components.

You operate components at clock rates above their normal design rate. Many motherboards now allow fine tweaking of clock rates and voltages for processor, memory, and graphics card.

I would agree. Often a cost effective, and notable improved performance can be obtained by upgrading or increasing the amount of RAM.

Cheers guys, the problem is I've upgraded my machine now to its optimum. My Graphics card is fine and I've gone as far as I can with RAM, However the games I play all need quicker processor speeds

I'll think on it some more