I wouldn't put any PC with a processor lower then 2.2ghz in a cab, that way you ensure all mame games that are playable work and you can also run almost any PC game like i do on my cab.
The Dreamcast emulater needs more then 700mhz as does alot of N64 games, if you're into console emulation and want to not be limited i would get a better PC. If it's just for mame it should work decent enough.
Goodluck.