What input method are you going to go with? Hangstrom, Keyboard Hack, Joypad Hack, IPAC?
I hear very good things about the IPAC but I didn't want to spend the extra money, and hacking a keyboard was a lot more challenging.
If your cocktail cab has been jamma converted, then you could use a JPAC.
I was going to go with the traditional cocktail cab design (ie facing), but decided that side-by-side would mean a much larger selection of 2 player games.
Have you visited
www.arcadecontrols.org?