If the cab will only be using MAME, integrated VGA would be fine. No real need for something more unless you need Svideo out, or want to play 3d PC games.
As for the processor, if you want new, I'd probably get an Nforce 2 based motherboard and something like a 2000+ Athlon XP or Sempron processor. I'm not sure if they make a 2000+ Sempron off hand, but you'd be fine getting the lowest Sempron available. They would probably be close to a 1ghz Athlon, but they will be a bit better more than likely.
Honestly, I would just find the nicest price point on a CPU and not worry about getting similar ones. A lot of times, its smarter to get a faster chip because the price diff is so low, etc. Just an idea.