If you want to make a jamma box like mine all you need is a +5 -5 +12 volt powersuply for an arcade machine (I got one new for 20 ukp. Look under Amusement Machines in the yellow pages), a TV or tv-monitor like a philips cm 8833, which can take a Red Green Blue signal (pc monitors are no good), a 28 way edge connector, an 8 ohm speaker, and some joysticks and buttons (I've even heard of people converting snes pads but I don't fancy that myself).
Inside my joystick. A neat job NOT!
My
latest addition, a trackball that allows me to play Centipede on my ( now
heavily adapted ) jamma rig.
Missile Command, and Crystal Castles are next
on my wich list.
Note the scart pinout doc suggests putting the game sound through your TV. I don't think this is a brilliant idea. just wire an 8ohm speaker to the jamma connector instead.