I'm considering purchasing a used car from out of state. Do I need to modify the car (cat perhaps?) to pass CA emissions? The car is from New Mexico, if that helps. What are the procedures involved in getting the car registered in California? Thanks in advance!

the best way to know the procedures is to check with DMV..

It absolutely flew though CA smog with a huge amount of room to spare. Once you get your smog certificate it is the same as registering any other car (they used to have a tax on out-of-state cars but thankfully that was repealed). If the car was made in the last three years or so it should have no trouble clearing CA smog. What kind of car is it?

'91 GVR-4 :D