A P0420 or P0430 doesn't mean the cats have completely failed, it just means they are not operating at their intended efficiency level. IIRC, a 5% drop in efficiency will trigger a P0420. Before replacing your cats, check your fuel injectors and fuel pressure regulator. Leaking injectors will not only cause the engine to run exceptionally rich, but will also kill cats just as a lean condition would, which increases combustion temperatures. Check for vacuum leaks, a higher than normal MAF voltage, etc. which also causes the engine to run rich. Check for any exhaust leaks between the front and rear oxygen sensors. A very minor leak is enough to throw a code.
Gently tap the catalyst and make sure nothing is loose/rattling inside with no rust or holes. Get a digital high temperature infrared thermometer and run the engine to operating temperature. At 2,500 rpm, check the temperatures at the inlet and the outlet. If the reading is ~100 F higher at the outlet, your catalyst is probably fine. At idle, this'll be about 30 F to 50 F.