[TurboRay]

 I was considering replacing the stock Cat which is beginning to rattle.

.
As you may know......a rattling cat-conv means that the honeycomb substrate "block" is loose inside its' "canister" and is usually disintegrated &/or broken into several pieces. And......those chunks/pieces can sometimes plug/restrict downstream passageways.....including those inside the muffler.

Soooo......if you don't intend to replace the cat right away, I suggest at least removing and gutting it with a piece of pipe and a hammer ~ then reinstalling it empty.  <shrug>

[Howard]

You have some options BEFORE you get your smog check.
You need to find out if your car is a CA car or a 49-state car. If it is a CA car, you will be required to have a CARB approved cat on it.
If it is a 49-state car, it can have an EPA/49-state approved....AS LONG AS IT IS A WORKING CAT!!!! If the testing shop says is has failed, you MUST replace it with a CARB approved CAT.
My car was a 49-state SVO, so I replaced the cat with a 49-state cat BEFORE testing.
The reason I mention this is that the CARB-approved cats are 2-3 times the cost of a 49-state cat!