My sister-in-law is a Ms. Pac Man fanatic. For Christmas, I'm trying to repurpose a 700MHz, 128MB computer as a MAME box playing some of the arcade classics for her.
I've found that recent versions of MAME are too slow to be useful on older hardware, even for games as simple as PacMan. I (somewhat arbitrarily) selected MAME 0.56 as my game engine for this machine and selected ROMs the 0.134 romset.
The games I've selected to include in the gift are:
-1941
-1943
-Dig Dug
-Donkey Kong
-Donkey Kong Jr
-Frogger
-Galaga
-Space Invaders
-Joust
-Jr Pac Man
-Ladybug
-Moon Patrol
-Ms Pac Man
-New York New York
-Pac Man
-Qix
-Super Pac Man
-Tetris
(If any of you think I've missed an obvious classic, say so!)
I'm having problems getting some of the games to work and would like the community's help. Remember I'm using new ROMs on an old emulator. I get errors trying to run Dig Dug, Galaga, and Moon Patrol. I certainly feel these three games are required for a classic arcade cabinet!

In all three cases, I get an error that "Required files are missing, the game cannot be run." Of course, MAME 0.134 plays all of these games; it just plays them in stop-motion.
Is there an easy solution to this? Is there a convenient source of old ROM files? If breaking changes were made to MAME, in which version were these changes made and which ROM set should I look for to find replacements?