I had the same thing happen to me last year (you can look up my posts on that if you wish). there is a very high probability that you will need a new cat. the O2 sensor is a lower probability. make sure you have new spark plugs, dist cap and rotor, an the vacuum reading are where they should be (no vacuum leaks)

Order your own parts and install yourself.

You may very well need a cat and an O2 sensor but the IAC will have no effect on getting your vehicle through smog. You may have other problems that the IAC will address but from the sounds of things you are doing a "loaded mode" test and the vehicles idle quality should not even come into the picture. Any idea how good of a cat you had installed last time. You typically get what you pay for in catalytic converters and some times you do not even get that. I can get cats done anywhere from about 75 bucks on up and the $75.00 variety is what we in the business refer to as the "clean for a day" model. If you had a bottom of the line cat installed two years ago it would be entirely possible for it to be dead today. Elevated NOx is caused by excessive combustion chamber temperatures. Typical causes of this are lean air/fuel mix or overly advanced or severly retarded timing. Make sure that your fuel filter is clean, you have no vacuum leaks, and that the O2 sensor is working properly. Obviously the easiest way to do this is to hang new parts where you can. Make sure that your vehicle is good and hot when you deliver it to the shop to have the smog test done again and put some good gas in it, not the 87 octane that we all run.

Mark