It's just not as simple as knowing what version of mame will run on your PC. It's like when someone asks me what king of PC/ laptop should they buy. I always have to ask first "Well, what are you going to use it for?"
You first need to answer what OS will you be using, since that can potentially be more of a resource hog then the version of mame (or intended games) you will be running. In most cases, XP will require less Ram and resources than win7, for example. I personally wouldn't run Win7 on anything less than 2GB of Ram to start, regardless of any other hardware specs. Core 2 Duo and above preferred,
But then you have to take into account any front end (HS will require much more recent hardware) and what type of games you will be playing. I have a couple builds in planning stages and due to needs in each cab, I'll be using very different PCs for these very same reasons.
A P4 with -just- 512mb of Ram will not get allot done, unless going with stripped out version of an OS and playing basic games in an older version of mame. If that's what you are looking for then great. But at some point you'll have to look at any future plans for the cab and determine if its worth it to go low spec on the PC now, if you'll just be going through all this configuration again in the not to distant future.