The 790 SFF case by itself is not worth much in value and a troublesome case to upgrade to new motherboard due to myriad technical difficulties including proprietary front panel pinout, low profile cooler, and thin form factor psu size limitations.  Much easier to get a complete used optiplex 7020 series pc that supports 4th gen Intel cpu, or build your own pc in a Dell or non-Dell mini tower case.
Have an Optiplex 790 SFF  i5-2400. Service Tag . Want to upgrade to i5-4570 or better w 16G Ram Anyone found any mobo's that will fit? (Dell or otherwise) 
Optiplex 790 SFF mobo upgrade