Hey MM,
I've had a little experience a while back, but certainly what I don't know could fill a warehouse!
Are you using PC screens or the original arcade monitors?
I ran mine with a P4 (2.?), 1 GB RAM and an ArcadeVGA card (as I was using arcade monitor).
JPAC interface card and a 100g HDD
The only purchase for the PC was the video card, the rest was old parts.
It ran the classics fine, but Sys22 games didn't like it (Rave Racer etc).