I'd recommend reading these forums a bit, as this topic comes up very very often. There is even a "official" what computer do I need thread around here somewhere.
To try to answer your question though, most anything will work. For one thing, no computer you can build today will run EVERY game in mame at optimum speeds, the processing power just isn't there yet. Think about that for a second and then realize that you can play 98% of all MAME games on something like a 1ghz system. You could also probably play 90% of the games on a 500-700mhz system.
So as you can see... most anything will work for you. To play the newer games that are actually playable, I'd say go for a 1.5-2.0ghz system, but only if you really want to play those handful of games... otherwise, you're wasting your money.
I don't use Macs but people have used them in mame machines before, so it is an option. Don't use ME, its the worst M$ OS ever created... in fact, throw the disc away right now..... no.... break it, burn it, then throw it away.

98 is great since it isn't as bloated as XP, but I'd only suggest it for an older system. If you are building a new system, you will have no problems at all running XP, and that's what I would suggest. Linux is also a good option, and works great (I hear) once set up. Again linux is especially nice for older systems, so if you have the ability, XP will probably be "easiest" ... but not necessarily "best"
I also prefer AMD, but like you said, either AMD or Intel will work great. Take the better deal, whether its AMD or Intel. One thing to note is that you probably don't "need" 512mb of ram if all you're using is MAME.... but it will help if you're gonna be playing anything other than MAME.
I have a pretty high end AMD system at home and it plays just about everything except the handful of games that are still unplayable today. For comparison, its a AMD 2500+ at 3200+ speeds.
Hope I helped a bit