I love the site, but the search function can leave a little to be desired and it seems that a lot of questions get asked over and over again. So I am going to start this post and maybe answer some newbie questions. Please pitch in, since I am kinda a noob myself:
Q: What kind of system do I need?
A: Depends on what you want to play and what front end you want. For MAME realize that the most important factor for performance of the games themselves is processor speed. You don't need a lot of RAM, and the only purpose of the video card is to export the video from the computer to the monitor. If you run a basic front end, you don't need much of a system beyond what is needed to run the games, if you run hyperspin or gameex or one of the other front ends with all the bells and whistles, then you need a better video card and more RAM to make them run smoothly
1) Arcade classics - For Mame, a low end PC (P4, 512k ram, cheap, not onboard video) will suffice to play 90% of the games you want to play.
2) 3d fighters, games from late 90's and 2000 - recommended you go core 2 duo. probably wouldn't hurt to have at least 1 gig of ram too. Mame is optimized for dual cores, you don't get much of a bump using a quad core processor. A 64bit operating system and 64bit MAME would give you the most bang for your buck.
3) How do I get Gauntlet Dark Legacy, NFL Blitz, Time Crisis to run smoothly? - You can't, at least not in MAME. Even if you have a top of the line rig, these will still run choppy because the emulation on these titles is still a work in progress. That being said, if you explore your options and use other emulators - dreamcast, model 2, zinc, you will find that these games run pretty well outside of MAME on even pretty average systems.
4) What about lightguns? - Depends on the monitor type. Make sure you are running a MAME version that is above .130. You pretty much have three options: Ultimarc aimtraks (CRT, LCD, projection on a wall, pretty much anything) , actlabs light guns (CRT only), and EMS Top Guns. Each solution has its plusses and minuses in terms of cost, availability, support and use.
5) Where do I get (name) rom? - NEVER ask this question on the forums. Ask a friend, or do what most people do and get a torrent client and use the old googler on the fabulous interweb machine you have.
What is the best monitor?
A: Depends on the style of cabinet you want, how much dough you want to spend, and how "authentic you want to be. If you want a slim cabinet, like the famous Knievel Woody (do a search for it, its worth it) then you need an LCD. Otherwise its up to you to choose between arcade monitor and CRT TV. My advice? Go for the TV that has S-video. They are cheap, easy to hookup and most people (non-obsessed BYOAC'ers) wouldn't be able to tell the difference between a well concealed tv and an arcade monitor.
General advice
1) buy quality tools - I bought a cheap router off fleabay. Hated it. Went and got a decent router from lowes, spent 3 times as much I love it. Tools are an investment, you may find that you actually like this woodworking stuff. If you hate it, you can just sell it on e-bay and recoup most of your cost. If you think its dumb to spend 200 bucks on a router you are sure you will only use once, buying a 200 dollar router, and then e-baying it for 150 is much better than buying a 50 dollar router you are stuck with.
2) Tools you MUST have: Circular saw or jigsaw, router, screw driver, wire stripper/crimper, workbench or sawhorses.
3) Be patient - take your time, make it look right. Be at one with sanding.
4) Paint/stain do not cover imperfections, they just make them a different color.
5) Buy used and fix it - there are TONS of threads and resources on how to fix monitors, coin doors, old cabinets. This can save you TONS of money. I got an Asahi Seiko coin door with coin mechs for 20 bucks off of ebay. A 5 dollar cup brush attachment and 6 bucks of spray paint, and viola, brand new looking coin doors with working mechs for under 35 bucks.
6) The franken panel is your enemy - The more button and controls you have, the less likely the casual person is to use your machine. Everyone loved the original Nintendo, simple control pad, 2 buttons. Why does the wii outsell the x-box? Because its easy and intuitive to use. When most people picture arcade, they picture 2 joysticks and a handful of buttons. As impressive as a 4 player panel, with 6 buttons per player, and a dedicated 4 way, and a trackball, and a spinner, and a analog flight stick, etc. etc. is to us, it looks like an alien, repulsive thing to most people. If you want to play all those games, build multiple cabs, when adding something to your control layout think about how much that control will actually be used, and if the added cost, confusion, space that it takes up is worth it. For me the joy of this hobby us building something that others will enjoy to play with me. If I build something and nobody uses it, its a tremendous waste.