And then if you get a new motherboard, why buy an old one, so then you'd need a new CPU, new RAM, etc etc . . . personally I think if you get a new board you may as well just buy a new computer, because that's what you'll effectively be doing anyway.
And to answer your unanswered question, Briguy, I would replace the Dell heatsink with a new one if you upgrade the CPU, but make sure it will fit in the case as Dell cases are not always standard either.