To be more exact, just about any PC can do the job on a level where you will have a great and fun machine. The newer games thing has very little meaning, because almost everything past the mid 90s wants specialty analog controls that you simply are not going to have. I don't actually expect anyone to go out and source a 15 year old PC. There is no need, people are throwing out 5 year old systems today that can run modern Mame versions like a boss.
Haze is very correct in saying that newer mame versions have lots more bugs fixed. It is great that this is still being done. When the PC in my mame cabinet gives out then I will upgrade my mame version to match the new PC, until then, the old software is still doing just fine.