I've got a Mac G3 computer with OS X on it that I would be willing to trade for some arcade parts, a dead/empty cabinet, or some PC-based hardware.
Here's a link to the description of the computer -
http://www.everymac.com/systems/apple/powermac_g3/stats/powermac_g3_266_aio.htmlThis particular one has 192 mb of RAM instead of the standard 32 mb, has a USB card in it, and has a 40x cd-rom drive instead of the standard 24x. It has OS X 10.2 (Jaguar) installed on it. The harddrive is IDE, so you should be able to easily upgrade it to something bigger if you'd like.
The whole unit weighs about 60 lbs, as it has the built in 15" CRT monitor.
EDIT: This would make a great PC for a kid's room, as it is very rugged and easy to use. It would also be great for anyone who wants to learn OS X, but doesn't want to drop $1000 on an iBook, only to risk finding out that they don't like Apple's GUI...
EDIT2: You could probably also run MacMAME on it and turn it into a countertop machine.