Despite the Gauntlet theme I want to be able to run some more recent games on the cabinet, such as Mortal Kombat, NBA Jam, NeoGeo games etc. I've also got a donor PC to use which has an Intel Core2Duo 2.66ghz CPU.
With this in mind I've decided on the Zotac 9300 mini-itx motherboard. It can take the E7300 CPU I have, includes an onboard Geforce 9300 GPU (and a PCI-E 16 slot), on-board wifi, a form factor of just 170mm X 170mm and when coupled with a 150w PicoPSU I can power it using an external 12v power supply.
On the downside micro-itx boards don't come cheap, I'm shopping around and expect to pay around £100 for the board.