Highly doubt it, but if it did happen they'd probably either:
1) Make their own version of a video card by ripping off either ATI or nVidia and calling it their own. Hey, it happened to Xerox!
2) MS will buy ATI or nVidia (but they'd have some serious anti-trust issues if they did)
By the way Gekko, considering you OWN an XBox you should be thinking a little more positive
Also, AMD was initially supposed to do the first video caard, but nVidia approached Bill Gates himself and the change was made. See IGN for the source.