The title says it all, I have a Intel® Pentium® 4 520 Processor 3.0ghz, and I want to play games like arma 2, arma 3, diablo 3, I also just bought a new graphics card but I need a new cpu to run some games.
Yeah sorry to let you know man but that needs to be completely rebuilt. You're likely better off building a custom PC than trying to upgrade something that doesn't even have PCIE. I highly doubt your CPU would even be the bottleneck and you wouldn't be able to get a good enough graphics card on that platform even if you were to overpay for the older style.
Ah, well the spec sheet I found online fails then. Again, those games are not overly CPU demanding... you will get way better performance by returning that card and grabbing something along the lines of a HD 6670 or better than you would by doing anything to your CPU.