Everyone is going to rush for the 720 just like they ditched the OG Xbox for the 360, remember how quickly people flamed the 360 for red rings and such? It took me 2 yrs before i bought a 360 maybe i will only wait a year for the 720 but I am certainly not going to be one of the first lab rats to throw away 500 dollars on a console that will probably have as many bugs and issues as the iphone.
Just to make a small point, its takes two years for them to make one of these CODs. The hardware has to be available to these compainies for them to be able to make the games, at least to some extent. Ive seen links to these companies hiring for next gen consoles, but only over the last 6 mo or so.... Might not see it this year...at least on 720
I'm not exactly sure how it works on the console, but if the PC is any indication, older games run better on newer pc's. If they port the game they are currently working on, it should run smooth as butter on the new consol, even if it is a port.
Most likely the new 720 will be released at the same time as the next CoD - it is afterall the biggest game release of the year.
Given that the next CoD will be designed to be played on the next-gen console the current 360 will either not be able to play the new CoD at all, or very poorly.