I think you guys are wrong here... are you guys thinking that I have Dell Optiplex 755?
But I have Dell Optiplex 755 SFF (slim version)
I am thinking to buy 9600GT so.. My uncle said something about Power supply, I wanna know what he meant.. He dint explained well... So what power supply will I need and will 9600GT fit on my Optiplex 755 SFF?
There's a problem there, your PSU is definitely not powerful enough, but Dell PSU's are specifically designed for Dell computers only to prevent you from expanding your system extensively. It's called BTX form factor. Getting one of those is more expensive than the standard ATX and are rare.