yeah, if they did a mobo revision the only thing you would need to get would be the mobo and the graphics card. The processor and Ram would all work fine in it. A board that accepts PCI-E will not also need a all new processor and stuff, that stuff hasn't changed at all. So, all Alienware would need to do would offer a upgrade to current owners to buy a mobo that can accept PCI-E graphics cards.
It would be really sweet to have a 256mb graphics card on my Roswell-m, and I would certainly pay for an upgrade to my motherboard as long as it is not rediculously priced. I would think somewhere along the lines of at most $500 would seem reasonable? That would be half of what they charge now if you have to replace your mobo if you are not underwarrenty. So, if you are underwarrenty it should be cheaper (or even free