http://www.xboxic.com/news/3980

To be known as Xbox Originals, the next Dashboard Update will allow certain smash hit original xbox titles to be avaliable for download on the Xbox 360 for 1200 MS points each.

The Dashboard update is due on the 4th of December, and more titles will be avaliable in the future. 

I think most original Xbox titles were on single sided DVD's for those who are wondering about size (meaning they'll max out at around 4 gig). I sure hope they put Ninja Gaiden on here, because I never played it. They probably will around the release of the second iteration.

I think its safe to say given the existence of this service, that Microsoft would have no problem with games like Goldeneye 64 and Perfect Dark 64 (not to mention the Banjos) being released on live, at least from a file-size perspective.