As the main aim of this site is to encourage people to play games made for simpler systems... The questions becomes, how do you classify simpler systems?
The best I've come up with is two main groups.
The first being to get the original hardware, or maybe a clone, then play the games as originally intended. This would have sub categories for things like original system, upgraded hardware, multi-title systems and the like.
The second group would be using an off the shelf or self built system. For these types of general purpose machines it's normally split into single board versions or more modular designs.
Though as the lines are blurring between the two I've wondered if maximum power draw could be a better subdivision.
< 2.5W level 1
2.5W - 5W level 2
5W - 10W level 3
10W - 20W level 4
20W - 30W level 5
Then a new level for each extra 10W.
It seems most future performance improvements are most likely to come in either a reduction in size or the amount of performance per watt of energy used.
Dose that make sense?