Uhmm FX line been dead for a very long time. In fact the FX-8xxx were 'just' in front of a i3-4xxx Gen
https://www.youtube.com/watch?v=gNt_KS0UUhI . So honestly your best bet (to reuse your DDR3) would be to pick up a i5-4xxx combo, most likely used, because once you go any higher gen OR Ryzen your stuck with replacing the RAM to DDR4.
This is the crux point, people are dumping (as they did with DDR2) old systems because they won't work with the new hardware.
Do a search on www.pcpartpicker.com for some pricing and such.
Don't forget a couple key things, changes to the Mobo and CPU like that most likely will need a PSU change to handle the more powerful hardware, so keep in mind what PSU you have. Secondly if this computer your modifying was a Dell, HP, etc. OEM, then you have to buy a new copy of Windows "off the shelf" (www.windows.com) and install it clean, because the OEM Modified Windows you have will refuse to work with any other hardware changes like this, it violates the OEm license.