Just a thought - it won't help you play Mame games, but hey there are lots of other games that are lots of fun to play on an arcade machine. Say, playstation emulation, N64 emulation, or just plain PC games. There are some really awesome racing games for PC that would rock with a steering wheel and pedals, plus games like Vice City can play pretty well on a cab. There was also a thread way back a couple months or so, where people talked about PC games that were basically arcade-style games with killer modern 3D graphics. My theory is you'll almost never regret spending a little more on computer hardware to get more flexibility.
Plus, since you're putting a high power modern system together, if it's near your main television and has a video input, you could use it as a digital VCR and record episodes of your shows to XviD files on CDs.
But, all that does complicate the cab a little bit - you end up needing an accessible keyboard, and maybe some cords going into the back too. My "arcade cab" (if it can be considered a cab) doubles as a Maya network render node, and helps me render my graphics faster, plays some PC games as well as Mame games, and I use its hard drive for networked storage.