Need help, trying to upgrade my graphics card

i currently own a HP Pavillion P6210y pc with the integrated nvidia geforce 9100 graphics card. its time to upgrade and im not sure what my best route is. I have a Amd athlon 2 620 quad core processor with a 6 gb system memory and a 640gb hard drive operating on windows 7 64 bit. im trying to get the best experience i can in the games i play.( wow, starcraft2...) New to this whole things so talk stupid to me and pls give me some referals to what u reccomend. Along with the graphics card if i need any other additional items like a new power supply or something pls make note of that, like i said i dont know left from right in this subject. Any help would be greatly appreciated. oh yeah i dont really have a budget but the less money the better