There is actually some substance to that "urban myth" about Dell and power supplies. While a standard ATX power supply will not FRY a Dell, it will probably not fit in your case; there is no ATX standard for the configuration of the fan/plugs on the back of the power supply, and Dell is notorious for having their own awkward configuration.
So, when upgrading/replacing your Dell power supply, you do have to make sure that it is Dell compatible, or at least visually check (at least three times, since you will be wrong the first two) that the plugs and fan holes line up with what you have in your machine. In addition, power supplies with switches on the back will NOT fit in Dell cases.
tldr: Dell sucks; triple-check that your new power supply will fit.