96 Camry Emissions / CA Smog Check

Hi.
I've been reading up on this site ever since I failed the biannual smog check mandated by CA's DMV.
I've done several things since- a tune-up, changed fuel filter, flushed fuel system, new air filter, spark plug. Those things helped a little.
My only failure is NOX emissions @ under 20mph. Before the servicing it was about 1100ppm. After it was about 800-900. Timing was checked, but it wasn't off enough to matter so much I was told.
A new oxygen sensor helped a lot, but still just short of passing. It cut those emissions in half, within 100ppm (about 500) of passing (430 is the maximum for 15mph. )
I was told that both the O2 sensor and the catalytic converter should be replaced, but was given an outrageous price quote in the 600-700 $ range.
I checked and it seems my Camry has the much smaller type of converter that looks like it just bolts on to the exhaust manifold right under the hood.
Does this sound correct and if so, is this an easy thing to replace? Would these two things most likely fix the problem?
All I'm trying to do is renew my registration, I'd like to do the repairs myself, and I'll most likely be selling the car within a year or so anyway.
Thanks for your time and I've learned a lot since this issue led me to this website!