Hp a6603w video card change?

I have a hp a6603w,and i want to upgrade my integrated video card,i guess when i put a graphic card in its gonna overtake the old one,i just want to know what kind of slot this computer has,and what good video card can i buy without changing the power source or anything else,give me links what i can buy for this computer,i want to run,l4d,l4d2,tf2 and stuff with a ease,thanks :)