So apparently somebody made a Halo game for the Atari 2600... with Bungie's blessing! It looks a little lame but the idea is cool, bringing modern games to ancient consoles.
I want to say it's one to one, but in all honesty I'm a bit fuzzy there. I can recommend JoyFreak's sister site AdminIntel - Admin & Webmaster Resource where there are lots for people of varying degrees of expertise with development, you likely can get a far more precise answer there!