Here's what I have on my 2 mame cabinet to give u a clue on what's needed.
1st one:
Athlon 1800+ (1.3G, me guess)
512mb pc3200
radeon 7500 with svideo out.
---Plays all games at full speed except - sfiii only at 70 percent, but still full playable. ki/ki2 run at full speed.
2nd one
Pentium 4 prescott 2.4G
512mb pc3200
radeon 9800 pro (spare I had around) connect to vga
--Plays everything at full speed including sfiii3.
If you had to buy, I would get at least a Athlon 3500 or high, or Intel dual core. This would leave room for future games. And BTW, if you need to do other emulation like dc, either my machine will cut it. Had sfiii on the Pentium 4 machine, but it was doggy slow, before the mame version was available.