I installed the airport card (Apple logo facing out, different than 'normal' Apple computers as I found out later). Pressed the power button... Nothing. No beep. No disk drive noise. No screen flashes. Nothing at all. And, both the cube and the monitor (an Apple Studio Display LCD) had power so it was not something stupid. If the card was removed it booted perfectly, but re-install it and it would fail to boot every time. Seemed like either a bad Airport card or a bad slot.
I called the AppleCare line and a technician spent about 20 minutes validating what I found (even tried booting with the hardware test CD but no luck), he spoke with an engineer, then confirmed my assumption regarding the nature of the problem. We have a local Apple store where I purchased the Airport card. They tried everything and reproduced what I saw. They took a new Airport card - same results. Took their test card and NOW it worked. Took a third new card and the Cube would not boot again. They ended-up giving me their test card.
The technician did not know if this was just a bad batch of Airport cards or a firmware change. They did seem surprised that so many cards would be bad in a row. To me this seems more like something specific to the Cube. Anyway, I did not find anything along the way to help and thought that I would pass this along in case someone else has a similar problem. Both the Cube and the Airport card now work fine together.
BTW, both the AppleCare phone support people and the local Apple Store technicians were very helpful. Pretty impressive considering they were providing support for used hardware and a $79 piece of equipment that I added myself!

