Hopefully the people who need to know what the "minimum" system is for classic gaming will find this post in the future.
I'm building a mame into an old Donkey Kong cocktail cab (all metal), and needed a system with a tiny power supply, tiny motherboard and 2.5 gig hard drive. Coupled with this system is a standard CGA arcade monitor for realistic classic gaming, plus an 8 way joystick, 2 fire buttons (the aluminum bezels hid a second hole in the panels), plus P1/P2 start buttons on the main player side.
I decided to go with AdvanceMame .87 and ArcadeOS 2.52 (latest versions). The mobo was an IBM with 2 PC66 DIMM slots, an AGP and 3 PCI and 1 ISA slot. Onboard 1373 audio (untested yet). It has a Celeron 366 processor, 128 meg of memory, DOS 7 (from Win98SE boot disc), plus an old ELSA 4 meg PCI video card (nVidia Riva 128 processor).
With this paltry set-up, I was able to get on average 2x performance (unthrottled) for most classic games I loaded (Centipede, Pacman, Pengo, Burgertime, etc..). The output of Advance Mame was always 1.00x1.00 scaling since the monitor matched the modes set up perfectly.
Only annoyance was the 10 seconds it took to reload ArcadeOS after quiting a game. I may add more ram and set up a ram drive to cache some of ArcadeOS/AdvanceMame to reduce hard drive loading times.
So, anyone looking at a Celeron 366/128 meg PC66/4 meg nVidia PCI video willing to run direct to arcade monitor should have no problems playing most pre 86 games, and probably a nice chunk of those before 1990.
Enjoy!