It seems that with the addition of compressed hard drive support for certain games (KI1, KI2, etc), you'll need the biggest, baddest PC out there with a lot of overclocking to boot. For the few games that require this power, it's simply not worth overspending on a PC.
I would simply suggest getting the most PC that you could comfortably afford. Today's mainstream performance PC's will have no problem running 99% of MAME games. With that, you'll never miss the few that you can't run. Also, there's always the possibility for an upgrade somewhere down the line when today's processors run like molasses in comparison