RTG will always be a lot faster than AGA unless that version of MAME is doing something really badly wrong. MAME was never designed for indexed palette displays, and certainly not for bitplanes!
You'll also need very old ROMs as the 68k version is prehistoric now in MAME terms, and the ROM sets have changed a great deal.
Yes, the ROMs I linked are suitable for the Amiga version (which is 0.35.12).
>Spirantho why did the other poster list the games that would work? Is there someone who runs mame on 68k aga?
Yes, I have, but it is a shame the 68K port of MAME hasn't been updated lately...
