As a side-order to Greg's post about the Xbox 360 launch titles, you may be interested to view Microsoft's list of Xbox games that will run on the new machine. There are over 200 on there and the company promises that more will be converted as the weeks go on. As you may already be aware, key titles are also going to be updated so that they're fully compatible with HD displays - the standard bearers being Halo and Halo 2 (go here to learn about the ins and outs of their conversion).
Already, members of the Xbox community are grumbling about the selection with Planetxbox360.com, posting an alternative list of titles that aren't currently supported by the emulator. For example, if you were hoping to play any of your old Burnout, Dynasty Warriors or Unreal titles on 360 you'll currently be disappointed. In stark contrast, however, fans of Toxic Grind, Shamu's Deep Sea Adventures and the mighty Barbie Horse Adventures: Wild Horse Rescue will be thrilled to discover that these titles have been targetted for emulation.
