If you want somthing that will run pretty much any Mame game, you might also want to look at Dells' Outrageous Deals page on the Small Business site. They have some awesome deals right now, even have a PC for a bit over 319 with a 17" Tube monitor (Celeron 2.4 I believe), or you can go for around 449 for a 2.8ghz P4 256m 40g, with a 17" flat panel. While that might not be what you want to put in your cab, maybe you can replace your main PC and stick the older one in the cab (assuming you're using an old PC right now). I would think though the 319 machine would work fine for probably 95% of all the mame games, only ones you might have problems with are the NeoGeo & newer games (and the ones that require CHD files, they won't even run on my 3.4ghz P4 and be able to play 'em).
AND anybody can buy from the Business site, you don't have to prove you're a business at all. I've been ordering from there for years and had them deliver to my home without any problems.