According to the specs found here, the 520 is only compatible with Pentium 4 and Celeron D CPUs. Also this spec sheet for the 945G chipset does not list the CPU you have as compatible.
I have an old Optiplex GX520 which has an Intel Pentium 4 521 2.8GHz CPU in it. I bought an Intel Core 2 Quad Q8300 CPU to replace it with. I did some research and concluded that it might work, I couldn't really find a whole lot of helpful info about my motherboard and other components because they're so old. I installed it and turned on the pc but, big surprise, it won't POST. Tha fans come on and the hard drive spins to life, but the LED in the power button turns solid yellow instead of green like it normally does and there's no display on the screen. I've updated the BIOS for my motherboard to the latest version and reset the BIOS/CMOS. I've tried reseating the RAM and reorienting it. None of this has worked and it still does the same thing when I put in the new CPU. It still works just fine with the old CPU. I realize this is a very old device and is not very practical to upgrade, but I just wanted to see what I could do with it just for fun. Any help or ideas are much appreciated! There is also a good chance that I am totally wrong and it just isn't compatible. let me know if more info is needed! Thanks!
Optiplex GX520 won't post with "new" CPU.