Care about your money?
Fastest (fx?) athlon is the best hands down... But you pay a LOT for an extra few fps.
The fastest 64 is the best price / performance that can run within a few fps of all games that can run on any processor.
Intels are great but more expensive. But they are on par with amds most of the time.
There a few drivers that are better on intel over amd. But those are almost all emulated in Zinc (that runs on either VERY well).
So if you have endless money, get an AMD FX chip, then if you have a lot of money that you don't care about get a top of the line (non-dual core) intel... then if you have some money to spend go AMD..
but in the end... BEST idea is just to get a good processor thats about a year old... the you will be able to play 98% of all games that mame plays on any system. So you might not be able to play them all... I'm not even sure which games can run only on the top of the line only.. .but you might be paying a few hundred for 10 games.... and in a year, you will be able to upgrade your computer to something faster with the money you save now.