in my previous reply i stated buying a z420 "SYSTEM" not a motherboard might be cost efective depending on the z220 motherboard cost/availability
i don't think a z420 will work in a z220 case as most current HP workstations use punched out I/O panels in the back of the case instead of the replaceable I/O panels found on most consumer systems and 3rd party cases.
a z420 system came with a 400 watt supply, with a 600 watt unit as a option
i don't own either model, so i can't comment any further on if the case or board swap is pratcial