Help?

Hi

In basic terms overclocking a component (I.E, your HD6750) will increase the performance of the component, overclocking involves increasing the frequency (clock speed) so that it operates faster.
Some draw backs of overclocking include increased power consumption, increased Heat and noise, You may also experience instability.

There's plenty of guides on the web regarding overclocking, Google is your friend.

Thnk you omgitzfatal.

And 1more thing when i play games my cpu cabinet's cover is heated very much.
Any reason for this?

Are you referring to the top of your PC case? If so that's likely because heat rises, so all the heat in your case will rise to the top, most cases will have fans in the roof to help expel the heat.

Whilst gaming, your GPU & CPU come under load, hence generate a substantial amount of heat which is dissipated by their respective heatsinks & hence the hot air inside your case make your case feel warm/hot.
As mentioned by omgitzfatal, "all the heat in your case will rise to the top, most cases will have fans in the roof to help expel the heat."
 

So what i will do to decrease the heat?

Which CPU are you using?
Is it getting really hot?
Anyways, download HWMonitor to monitor your temps.

My cpu is of iball.

No fan in it.

The cover is getting too hot when i m playing games.

The case is iball
CPU= Processor. So, which CPU are you using? Intel?

As I mentioned before, download HWMonitor & post your temps.
If your CPU, GPU or HDD are getting too hot, then you might have to invest in a case with good airflow.