What is the difference between the various joystick options offered with the slikstik? The button on top is pretty obvious, but what about the "12 way" and the others?

The 12-way refers to, I believe, the Ikari Warriors style rotatable joystick. Used on a very small number of games.

Is there any place one can go to determine which controls were used with which games?

There's a util called SortInfo which allows you to import MAME's listinfo.txt file and filter out games based on controls, display, etc. Should give you what you want.

What are the minimum computer requirements to run MAME at a high framerate?

Depends on the gmaes, obviously. If you want to play the more taxing games (some of the Street Fighter clones, Neo Geo games, NBA Jam TE, etc.) you need something along the lines of a 1.2 or 1.4 GHz machine or you'll get bad framerates. I have an 800 running @ 900 and the upper-echelon games aren't very playable.

Dunno anything about the SlikStik, I was going the home-grown control panel route before my MAME project got indefinitely postponed due to lack of funds.