Hey, first thing I can tell you is you should go buy the book "Project Arcade: Build Your Own Arcade Machine". It's a fairly easy to read book on how to convert jamma cabinets to Mame cabinets, and it explains pretty much everything else on modifying arcade games. It was written by a guy on this forum, and it's like twenty bucks on Amazon.com. I wanted to build a new control panel for my arcade game, having no experience whatsoever cuttin wires, wood, or anything electrical. I bought the Project Arcade book and it showed me eberything I need to know.
Generally, the people on this message board will help you if you give really specific questions. Some of the people here arent quick to help out the new guy. So go buy the book, it should explain basically everything you need to know.
In response to exactly what you would need to convert your cabinet, you should go take a look at the ArcadeVGA video card as well as the Ipac board on
http://www.ultimarc.com.
The ArcadeVGA replaces your Mame PC's video card. It outputs at resolutions that are safe for arcade monitors. The Ipac lets you wire your arcade pushbuttons to your PC through your Keyboard jack, so for example when you press the "Fire" button on your arcade control panel, Mame on your computer will interpret it.
The Jpac combines an Ipac + video, but from what I understand the ArcadeVGA is safer for some reason. Im not sure if the Jpac can output resolutions that can damge your monitor, or what. You should read the descriptions of both, and ask some of the pros here before you go spending money.
Anyways, take a look at that stuff and good luck.