[bepperb]

Usually the throttle plate gets cleaned when you clean out the idle air control valve, and on newer cars the maf sensor. Basically, the PCV spills crankcase gasses upstream of them, and over time they can get gummed up with deposits. Do some searches on camry IAC valve cleaning and you'll find what KathyRicks is talking about, but I'm not certain it applies to your exact situation (but probably worth taking a look at with your mileage and current problems).

I think you're on the right track of replacing the o2 sensors first. Likely that's the cause. If the cat is bad usually you'll get significant power loss at mid/high rpm and rotten egg exhaust smell, neither of which you've mentioned. The 420 is a catch-all code, though, so you never really know.