- the cab has the wonderful 6 button layout for each player that I had wanted, and I know that I wanted to add a button underneath for inserting coins, but I'm starting to wonder now how many more buttons, if any, I will need. I'm thinking an escape button might be useful, and maybe a pause, but I also don't want to get too button happy. I'm using Hyperspin for the software and see that most of it can be done via joystick, although, and correct me if I'm wrong, not able to escape out of a game once it is running.
If you have an encoder that supports it (I-Pac, KADE, KeyWiz), the "shift button" approach that Felsir mentioned works great especially if you want to keep it looking more like a regular cab than a MAME cab.
You can also wire the coin slot switches to the encoder to accept tokens and some people have also mounted microswitches behind the coin reject button so that you can press the coin reject to add a credit if you don't have a token handy.
The question is what admin functions do you need to be able to do?
The main ones are:
P1/P2 Start (Dedicated buttons on panel)
P1/P2 Coin (Shifted function, coin slot switch, coin reject switch)
Exit (Shifted function or esc)
Pause (Shifted function or p)
Select (P1 Button1 or enter)
Menu (Shifted function or tab)
The only down side to using shifted functions is accidently activating them during game play, especially 2 player cooperative games.
P1 Start is usually the shift button and P1 Start + P2 Start is usually exit.
If you and a friend both try to start at the same time, you exit.
If you use shifted functions, be sure to include instructions on the bezel.
- for the pc, I've read that the graphics card doesn't matter, or maybe it does, and that MAME really uses system RAM for most, if not all, of the processing. A lot of this depends on the age of the post. If I want to run newer games, is there benefit to a decent graphics card, and how much RAM is enough, 6gb, more?
If you aren't going to add Visual Pinball/Future Pinball, you won't need a graphics card.
6GB of RAM seems like overkill, but if it's what you already have in there . . .

I have a 19 or 20" one that is not widescreen that I have already, unless there is benefit to using a widescreen monitor.
The original games weren't on widescreen so a 4:3 monitor (non-widescreen) is the way to go.
With a widescreen, you waste the extra width and vertical games look really puny.
Exception: The Ninja Warriors by Taito (1987) used 3 screens to make a widescreen

are there any tips/guides that I did not see to building a shelf or somehow mounting it, preferably without decasing it? I still have the monitor braces in the cab, hoping I might be able to incorporate that somehow.
Look for 4 holes on the back of your monitor arranged in a 100 mm square. These are the
VESA Mount holes.
Sometimes the holes are covered by a sticker.
Mount the monitor to a board then attch the board to the cab using angle brackets or cleats like this pic that Edekoning posted.

If the monitor is older than ~1997, it probably won't have mount holes and you'll need to make a shelf to hold it or use some kind of brackets to mount it.
Scott