Last I checked there are no reliable emulators available for xbox. Steam games would be the biggest concern for specs. Fortunately building a decent computer for a small amount of cash is possible.
This link here is probably the cheapest you could swing it while using modern parts. As for the video marquee, I have an idea. But your cabinet dimensions are not friendly towards it, and the solution you might find will probably be very labor intensive and expensive.
Currently at work, but for the marquee I can only point you towards this seller. I have not researched this too much myself, once I get to my desktop I can probably go into heavy nitty gritty on the details. With some hard google work you may be able to find a screen that will accommodate your cabinet dimensions.
http://www.ebay.com/sch/vitrolight/m.html?_nkw=&_armrs=1&_ipg=&_from=
Another edit: Just found this site, I assume you are aware of it also. http://www.paradoxarcades.com/lcd-marquee/
