It is a function of the chipset used on the motherboard and what CPUs the BIOS is compatible with. Those systems are simply not compatible with the more modern CPUs.
Also, since that model of Dell uses a proprietary BTX format motherboard, you can't just change the motherboard to a standard ATX model *like those used in nearly all systems).
To upgrade, you really need a new system. Sorry for "not so good" news.