Should I upgrade my Processor or Graphics Card?

Hey I got an AMD Athlon X2 3.2 GHZ and a Geforce GTX 450, and 4gb DDR 1600 memory, It sruggles a bit with smoothness in some games although load times are pretty good, Should I upgrade the Graphics card to something like a Geforce GTX 560? I wanna play the new Duke nukem and dont know whether i should use my cash on a new procesor or graphics card,, Im thinking graphics card??