Again, thanks for the info. I'd still love to hear as much advice as I can from others who have done this. Very glad I registered, it already seems like this is going to be a very helpful place. I'm used to console forums, and those people can be downright rude. 
This is one of the friendliest and most helpful forums you will find.
The key to a good arcade cabinet is planning first so you don't have to rework later.
For you - Step one would be finding out what games are out there and what you want to play on your cabinet - Are u mostly into 80's games - not much processing power required. 90's and up - a little higher requirements.
Step two would be whether you want to stay Macintosh, or go PC. Related to this would be whether you want a dedicated machine for the arcade cabinet, or try to have gaming and computing on the same machine.
From here, you can see which way to go - BTW, if you choose to go the PC route, it doesn't need to be super expensive -
A used Pentium III 500 or Duron 600 system will play all the 80's games and be around $300 or so.
In new components, check
www.pricewatch.com, but an XP2400 and motherboard is currently $78 (then you would need to add a cheap HD, video card, monitor, Power Supply and keyboard.)
Or complete Duron 1.3 systems are around $200 new without a monitor.