If the Gpu temperature reaches above 80 C.. is it going to harm my graphic card despite playing for 3-4 hours non stop?

One more thing.. Does this happens with every card that It gets hot and The temperature gets high no matter If it's 780 ti (an expensive card or GT 630 (a Cheap Card) ? and can I loose some temperatures by putting an Air Cooler for Cooling The GPU? and how much an Air cooler is Important?
 
First of all there are cards with stock cooling and cards with custom cooling. Then some of those cards with custom cooling have a factory overclock that you need to consider. You can always look up the suggested base temperatures for your GPU. If it is a custom cooled model, google for some reviews as they usually include temperatures.

Of course different cards operate under different temperatures. The AMD R9 290x is infamous for becoming incredibly hot for example. The GTX 780 and 780 ti models become pretty hot. I think something roughly something between 75-80 °C. I have an overclocked 780 with custom cooling and it usually doesn't exceed 70 °C.

Temperatures also depend on the cooling you have in your case. A case with good airflow and good coolers will carry the hot air out more quickly and your GPU will stay cooler too. You can put custom cooling on your GPU if you want to. It will run cooler and you might be able to overclock it. The GT 630 is incredibly weak though. Instead of tuning it, I would rather get a decent gaming GPU. Even the $200 models are several times as fast than the GT 630. A 660ti scores 6,5 times as high on http://www.videocardbenchmark.net for example.
 
Yeah, You're right that GT 630 is very weak but I'm not a hardcore gamer.. I just play games because from it you can be Intelligent and Your English will Improve! (As I've heard from people) 😀 P.S only If I could afford the 550 ti I would've bought it but no money=no gaming comp so I had to buy this crap -_- and kindly suggest me a cooler (A Quiet One) not a loud one.. it gets really annoying while you're not even playing games and still it gets loud as hell.. and My GPU's temperature doesn't exceeds 80 C.. I mostly play at 70 C..
 
I can't really help you with a GPU cooler. It is more common to get custom CPU coolers than GPU coolers. I have never bought or installed one. Usually you just buy a graphics card with a good cooler already installed. Somebody else will have to help you out there. Since your temperature is ok though, the only other issue is noise. You should make sure that it is you GPU and not your CPU that is too loud for you, so that you don't change the wrong part.