Well, al, I'm sure that both Dell and HP would LOVE to know when they became the same company, because they are not! Maybe you are thinking of HP and Compaq?
But you are right about buying a Dell or HP/Compaq regarding the motherboards and case issues. Also, as I hinted, the mobo's do not have all the features of one you would buy separately for a self-build. You can find adding things is almost impossible unless you remove something first.