It wouldn't matter if dell used btx, he wouldn't be using the case, powersupply, or anything else along those lines.
But you can keep the ram, don't see why you wouldn't unless it's a different speed than what you want to upgrade to.
Also, I would get the case/psu combo if you're really into the case; Antec uses their good powersupplies even in their case/psu deals, so it would be a quality powersupply.
Be sure that the hard drive will work (being sata, ultra ide, etc.)