Your processor is below the minimum requirement, so even with a new graphics card you probably can't run that game. Also, the power supply is only 300W, so you probably won't be able to run a graphics card that can handle a game as new as Diablo 3.
I don't think you have much of a chance with that system.
Oh crap lol. I meant diablo 2 and I also epic failed to mention that I do have a new power supply. Sorry.