the throttle body does half the job of the carburator, it controls how much air enters the engine. buying an aftermarket throttle body will allow more air into the engine and therefor more fuel because it is less restrictive and it's larger. if you only buy a throttle body you may not notice much of a difference in power(if at all, maybe a couple of horsepower) but if that is what is restricting the airflow you will. i'd say first install a cold air intake and new filter, then install a throttle body, then get your heads ported and polished or get new ones.
A catalitic converter is very restrictive on the exhaust, it's full of tiny platinum (or another catalist) beads that help reduce harmful emmissions. installing an aftermarket one might help if it is less restrictive, but you might fail your emmissions test. of course you can have the stocker installed when you go for drive clean and then change it (i am not condoning illegal activity i am just saying what could happen). also you might notice more performance gains from other mods (headers, cat back exhaust, etc) then you would if you didn't install the less restrictive cat (or just run a test pipe and not have a catalytic converter at all but that's bad for the environment and i dont suggest you do it)
hope this helped