The right CPU for an upgrade???

Hello everyone, I need a quick assistance on upgrading a HP small form factor dc7800. I would like to change the current CPU which is a Intel Core 2 duo to a Intel core i7 and also I would like to add an additional internal hard drive about 3tb when I get the money Lol. But for now my main concern is upgrading the CPU. Please direct me to the right CPU, say about 3.00 GHz. Finally, I am currently running windows 10 on a 32 bit system with 2Gb of RAM and 250Gb hard drive, if that info was needed.

FYI- I have been searching Ebay but don't want to buy the wrong CPU.

Thanks,
~Phil