This article on developing new (but so minimalist it might as well be retro) game hardware is interesting; down the page a bit it goes into some detail about handling video on restrained hardware. The base unit is an 8051-family chip intended as a controller for wireless keyboards.

http://www.adafruit.com/blog/2012/12/05/how-we-built-a-super-nintendo-out-of-a-wireless-keyboard-sifteo-sifteo/