Dell Optiplex 990 CPU Upgrade

Hi there! I have a Dell Optiplex 990 that came with a i7-2600. I've upgraded the CPU fan and case fan, as well as the RAM and graphics card, and I've installed a 500w PSU. Initially due to my budget and needs I decided I would just keep the cpu but now that I have a bit more saved up, I'm thinking of upgrading the CPU.

My question is, with the above specifications, what would be compatible with my system? Is there anything that definitely would NOT work? I'm pretty unfamiliar with a lot of tech stuff so I'm not sure if there are certain CPUs that wouldn't work with the motherboard or whatever. I know for the upgraded fans I got, I had to also get 4 pin to 5 pin converters which causes a startup error every time but otherwise doesn't cause me any problems, but I don't want to shell out money for a CPU only to find that it doesn't work with my system.

Thanks!