I have checked it out Dell does not allow you to upgrade unless you use their engineers, mine will allow me to fit a memory cards but not a drive, though as all my old hdd are now in external enclosures this isn't much of a worry. I will wait a while before fitting my old NEC 2500.
I think it has to do with them covering themselves for people who are just starting from damaging the hardware. I suppose what they mean with voiding the warranty is that if the person damages the mobo for example fitting the drive, then the company will not replace the mobo for free as they would if it was faulty or developed a fault. Therefore they would replace but all costs would have to be met by the person.