FWIW, I made a mac-based bartop from a PowerBook G4 1GHz. It is not as arcade-authentic looking as you can make a PC to be - but for my Dad it is good enough.
I ended up using Mame OS X and emulauncher. I made a desktop to match my emulaunch background and set emulaunch to load at startup. At boot you see the Apple logo, then emulaunch launches. Once you select a game you see the menu bar for a few seconds while the game loads. Likewise when you exit a game. Other than that the experience is pretty seamless.
Another downside, though, is the G4 is not capable of playing everything through MAME OS X - even some of the classics. No Donkey Kong or Joust, among some others. You can configure emulaunch to use MacMame, like Blanks suggests, which I imagine will be better at some of the classics with the older romsets.
Here is my bartop.