Also, if you are ever building them just to sell, then forget about horizontal monitors or Streetfighter layouts. I haven't done a whole lot of those to sell, but even when I did I felt like a total tool when all the buyers cared about checking out was Ms. Pac-Man, Donkey Kong, and Galaga. I decided never to build another horizontal one to sell again after the buyer for the last one came to pick it up (fast computer, horizontal monitor, trackball, streetfighter layout), and all he cared about was Pac-Man and Donkey Kong.
If you are building to sell build 4-way, vertical, with 2 buttons. Populate your gamelist with games that use those controls (there are like 94 of them). You will only need a 350 mhz computer, and your buyers will be happier with it. The 4-way games are what they REALLY want and building anything else is going to cost you more and provide them with a sub-par product.