Actually - I'd suggest getting the monitor first ...
I built the control panel first, spaced it out nicely, took my time, etc.
Problem is that unless I come up with a huge 27 inch monitor, my player 1 and player 2 joysticks are waaay too far apart.
I'm using two 21" crts, side by side, for modular purposes, so a cab is out of the question right now, but if you want a cabinet, I agree with JD. Get the monitor(s) figured out first. You will then have a good idea of what type of spacing you have and need for the control panel. Also, you can spec the cab accordingly so that it is proportionally appropro.
Or just use a scrap piece of wood and rig up the control panel. It's funny how much good xp comes from doing these things more than once. Leave a Little bit extra length than you think you need on all of the wires and you will be okay even if things change on the final setup. It's true though. You might think twice about the cabinet if you get everything up and working. I built my setup in 2008, and am still stuck on just the control panel. Those monitors are heavy to keep moving around and reorienting, but they can do it all with all of comfort of customization. Problem is that it lacks any semblance of polish or finish and are just a bunch of components on a backup dining room table in my basement.
Short answer: "Get the monitor first"