ya it all depends on what you want to gain. remember you cant just get horsepower etc from no where. if you gain something like top end power, chances are yoiu are losing a bit of lower end torque.
a few basic mods that you want to start off with is an exhaust and intake. Cold air intake (CAI) will get you more gains. you can put a free flowing cat converter, but it will increase the noise a lot. also, some cars already have a pretty free flowing converter, so by changing it you wont be gaining as much but your noise will go up greatly. you would want to put a free flowing muffler for sure, as stock ones are generally real restrictive. but remember that if you are puttiong an exhaust you want to make sure you can get enough air in as is going out, so that why you need an intake. also you can change the size of your piping of your exhaust. you wouldnt want to go more than 2.5 or 3 inches, because you would be losing a lot of power then.
if i were you, i would read in some honda or prelude forums online and see what people say about whats already in the car and what mods should be done and what to look out for.
for example, on my acura legend, i read in the forums that my piping is already 2.25" which is a pretty good size and the cat is already pretty free flowing and it wouldnt improve my performance too much by changing it. so all i did was change my muffler and i put an air breather to suck in more air. also, there usually ppl who show how to do custom mods for your car. im planning on doing a custom cCAI soon as shown on the acura legend forums. just make sure u research and figure our wat you want to do with your car and then see the gains and losses associated with certain mods.
my advice to you is dont change the cat, iunless you want the absolute max horse power you can get out of your car, because the gain is not worth the great amount of noise that will come. so for now, get an intake and a free flwoing muffler. and make sure you shop around different muffler shops. also, always get a written qiuote on their business card or something so when you go back they dont change hteir price on you.