You can never go wrong by getting the best of everything. If you're on a budget, the best advice I can give is that the CPU is everything when it comes to MAME. A $90 video card will run the current games every bit as well as a $600 video card. If you had experience, I'd suggest getting an i7 4790k, a liquid cooler for it, and overclock that sucker.
Just keep in mind that there are plenty of games that will not run full speed, but it will run the vast majority perfectly.
Just broke my personal record for number of consecutive days without dying!