Something that might not have been touched upon, sure you can get an old pc on the cheap and you will be able to play most games, and this is exactly what I did, except in my case I just used my old pc and built myself a new "main" pc.
In any case, it plays most games beautifully, but using Mala as a front end is extremely slow! Of coure I am using Mala with glitzy graphics and a gameplay avi video whenever a game is selected, but it literally takes about 3 seconds to browse through each game in the list.
For me, part of the charm of playing the cab was having a kicka55 front end that will motivate me and others to play games we've never heard of before. So I am going to end up buying a better computer for it just for that reason. Lucily it didn't cost me anything the first time around.
Of course that might not be your case but thought I'd pass that consideration along.

For reference my cab's pc is an AMD Athlon 1.4 or so with 512kb RAM and a Radeon 9600 with S-Video out.