dell optiplex 9020 MT cpu upgrade

This forum thread should help. In short the 4790K works but only with the optional 365W PSU. Since you have stated that you upgraded the PSU, you should have no problems.

I am loath to say that 365w psu requirement for i7-4790k is incorrect imo.  i7-4790k and i7-4790 have nearly the same TDP (88w vs 84w), and you cannot overclock the -k cpu anyway in 9020.  Therefore you can certainly run i7-4790k using the stock 290w psu in 9020mt.

like JOcean said it is not relevant in OP case since psu was already updated.  For other users thinking of upgrading cpu to 4790k, there is no need to unless you want to support a gpu that requires 6 or 8 pin PCIe connector.  That need would be for such gpu and not the -k cpu.

if the forum thread said 365w psu was required for 4790k, it was claimed by someone without evidence.

i have the same system. it's amazing. but I want to upgrade it with new one because i like the latest tech.