ys the hd7750 is the best option .. i have the sapphire hd7750 but it is not quiet edition. its a standard edition. doesnt go above 60 degrees even under load. so get the fanned version. gt640 is very inferior to 7750 because it uses ddr3 memory instead of gddr5 memory which 7750 uses
Your build is cheaper than a Console but still better. Amazing.