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