I disagree with the 600 mhz....I'm running a 750 mhz and a few of my games run choppy, but on my Duron 1000 mhz they run fine.
Sounds to me like you already have two computers

I think Paige is talking about only old school vertical games (I run them all fine on a 233). Get a slower one to run the classics (mosty vertical) and use your faster computer to run the newer games (mostly horizontal)
the one computer split among 2 cabs is just not worth the trouble when a cheap computer can net you 2 fully working mame cabs. I would just bug me to have a whole game sitting there unable to be used because someone is playing pacman. Kinda like that little space on the dash of a new car that just keeps telling you "you were to cheap to get the option that goes here" (man I gotta get back to therapy)
good luck whatever direction you decide