document updated 15 years ago, on Sep 3, 2008

console emulators (wikipedia, emulator-zone)

system	works/practical?	avg ROM size (compressed)	all ROMs (compressed)	top-n lists	problems / comments
NES	Yes	0.1 mb	<1 gb	[1] [2]
SNES	Yes	0.75mb	<1 gb	[1] [2] [3] [4]
Sega Genesis	Yes	1.3 mb	<1 gb	[1]
MAME	Yes	3 mb	14 gb (3 DVDs)	[1]
GBA	Yes	4 mb	4.8 gb	[1] [2] [3]	All ROMs fit on one DVD if you toss the 900mb of excessively compressed GBA Video titles.
N64	Yes	16 mb	3.5 gb	[1] [2] [3] [4]
NDS	getting there	40 mb	16 gb	[1]
Dreamcast	mostly	150 mb	too much	[1]	some games run too slowly
PSX	Yes	400 mb	too much	[1]	It can be a pain to download all components needed, so instead download a torrent with all components included (ePSXe, )
PSP	No	500 mb	too much	[1]
Game Cube	maybe	800 mb	too much	[1]
PS2	No	2800 mb	WAY too much	[1]	too slow

There are two things that make emulators have problems:

The original console had a very fast computer and graphics card. Emulation always has a LOT of overhead, so it takes a fast computer to emulate a much slower computer. So as home computers get faster, we'll be able to play newer generations of consoles, but it will always lag two or three generations. (this tends to make whole games "jumpy")
The original console was very complex internally, and emulator programmers haven't worked all of the bugs out yet. Newer consoles are orders of magnitude more complex than previous consoles, so this will only become a harder problem in the future. (games tend to start up and work mostly okay, but have periodic "glitches")