I have a small box of video cards, RAM, processors, wifi cards, etc that I was considering posting on eBay. Maybe I should post here first for cases like yourself! I even have an AGP video card (alongside PCI-express). Non of this stuff is expensive, so if you have a motherboard laying around that is a little newer, like 2006 or newer, I may have lots of parts.
With putting so much time into the cabinet, don't shortchange yourself TOO much on the computer, though you can always go overboard. I had a Q6600 quad core in a PC that I'm no longer using as my main, so it became my MAME computer. It is only 2.4 GHz, I think, and has a GTX 470 video card. Won't play CHD or Gradius III (or was it IV), but it plays most, which is what you're aiming for. I just bought an Intel G3258 to overclock (a very popular MAME CPU) as they are not expensive and with the right motherboard (there are forum posts on the internet), you can overclock to 4.6 GHz safely.
ANYWAY, my post was a vomit session of info ha haa. But if you need a cheap video card or a little newer parts after all, let me know. I won't charge much.