It would probably run MK3 alright. To give you an idea, using a version of Mame in the .55 - .61 range, I had a 733 mHz Celeron that ran MK1 and MK2 fine, and would run MK3 but with noticeable slowdown (though still somewhat playable). When I got an Athlon 1800+ (1.54 gHz), it ran MK3 with no problems.