The most important thing is CPU speed, so get the fastest CPU you can afford. I use an i7 2600K at 3.5Ghz base speed (with hyper-threading on) and i have no problems with all MAME games i tried so far. K type CPUs can be easily overclocked beyond 4+GHz so if you plan to overclock also get a decent cooling solution.
So far i had no need to overclock and i use many demanding emus like PCSX2 and Dolphin besides Mame.
Some Mame examples (at max speed without overclocking):
Mortal Kombat 4 - 150%
Tekken 3 - 210%
Street Fighter III 3rd Strike - 920%
Radical Bikers - 140%
Carnevil - 220%
Gauntlet Legends - 150%
Just don't expect to play any Naomi games at it's current state of emulation...
