I think that building a PC is something that everyone who is interested in should do. A hole punch in the geek card if you will. I also think that there while a person can quickly get carried away in over configuring, this is a hobby where many people can come in on an equal footing. When asked what do they need to build a good PC [for X] my answer is always the best you can afford. A slow computer is as useful as watching scrambled cable channels.
My larger point is that if you budget well, you can either purchase a premade PC from DELL or custom configure your own PC and you can use it for ANYTHING you want. If you add a front end that is more demanding, you'll be prepared. If you move on to virtual pinball, you'll be prepared.
Also what may start as a PC for emulating arcades, my turn into a console emulator, some of which are more demanding. In short, what starts as a MAME PC may not end as a MAME PC. A PC that is really good now, will end up as a PC that is good in a few years, where as PC that is good enough now to run MAME, may not age as well.
We don't need hexacore i7's with OC'd titanium video cards in SLI & 16gig of RAM to play MAME 37b5 roms. But it that is what you do, we can most likely rule out any problems with hardware.
So know your budget, know your computers intended purpose, and budget around that.
I reserve the right to revise and extend my remarks, though I don't anticipate the general nature of my statement change.
[/soapbox]