Craig,

Maybe video needn't be decided to "strictly". Decide an interface. How would the central '51 processor write to video?

With this flexibility, a single chip '51 video display (which would be a lot of fun and challenge) would be just as approp as a VGA approach.

Now if the need for a single approach was desired perhaps a decision regarding two issues would help:
1) Target monitor (TV or VGA)
2) Color/Mono.

In a way the question is do you retain the "Atari" or adopt a "PC" metaphor.

Basic greyscale to a Television target is pretty easy and their are many examples extant. This could be done with a single '51 variant but keep in mind that done this way you have no more screen display than you have integrated RAM. Why build a character display when bit mapping extends the functionality of even the lowest of resolution displays?

With external hardware (counters/comparitors/SRAM) things are much easier. Color agility is do-able but more complex and would require external hardware "end of song" for a TV target.

VGA monitor interfacing is again pretty straight forward you provide the H-sync (~34kHz)/V-sync(60/72Hz)
The pixel rate (co-ordinated with the sync) will be what ever rate you can shift pixels out. A single Par/Ser chip reduces the u-controller requirement nicely.

regards,
p