If you are running a dvdrom without a decoder card, there's no reason to think you would need one for the dvdrw.

Also, once you have the dvdrw, there's no reason to upgrade the cdrw as the dvdrw will burn cd's as well. You need to read the specs carefully - the dvd burning speed is on a different scale than the cd burning scale. For example, my dvdrw burns dvd's at 12x and cd's at 48x.