The k&n filter as mentioned above is the easiest and cheapest. Then if you can find one get an Engloid plenum and mate that with a 240sx throttle body. After that port and polish the heads. Of course while doing this you'll want to get all this air out so you'll need to get headers, high flow cat, and larger exhuast. On a side note skip the larger throttle body if you can't get an Enlgoid plenum. The throttle body alone doesn't do anything. Then again probably the easiest way to get air in the engine would be a turbo.
I can't really afford a turbo considering I'm about to go to college and I'm working part time at a burger joint haha. I was thinking a good filter would do. I also like the thought of a larger exhaust and high flow cat, but then there's another issue with money. Does anyone know anything about RAM air? I'm completely clueless as to what it is and if you could even (or if I even would want to) put that on a 87 300zx.
2nd ram air would be kinda pointless on our cars, especailly an NA. Just stick with a new air filter,new cat, and muffler. And on an NA the type of muffler you get isnt going to make much of a difference, just get a fairly cheap one that looks & sounds good.
Oh and just so you know look to get about a total of 7hp from intake and exhuast together. If money is really tight you may want to consider if 7hp is worth the 250 it's going to cost you to do the intake and exhuast. Probably your next questions are going to be where can I find the exhuast system and what kind. Just go to a local muffler shop and have them custom make one for you. Ask for a mandrel bent 2.5 inch exhuast system. You can get a crushed bent exhuast system made for cheaper but mandrel is much better so you you might as well do it right the first time. As for the intake get a k&n cone filter, I like part #RE-0930. You can go with the longer filter as shown in the picture above but there really is no point. What ever filter you chose just make sure it fits and that the hood will close without crushing it. After you have the filter just get a 3 inch diameter pipe that is about 8inches long. Remove air box and resonator and then place pipe and filter in place. The length of the pipe doesn't really matter so just measure it to how long you need corresponding with the filter you got. What you get is an intake system that gives you about 2hp and an exhuast system that gets you about 5hp.
The resonator does as it's name says. It cuts down on the resonance made from the engine sucking in air, baiscly it's there to quite things down. Thats why most say when they install the intake their car makes a growling type sound when they rev the engine. From what I've read it doesn't restrict flow so it's up to you if you want the noise factor or not. I honestly just went back to the stock set up with a K&N drop in filter and didn't notice a difference in performance. I think the main gain of hp from intake comes from the better breathing filter rather than cone vs air box.
This is probably not what the OP wants to hear:
Save your bucks and don't piss around with mods on the N/A motor. Save up and get a proper turbo car with the fatter brakes and 40 extra hp and stuff. It's cheaper in the long run.
Maintain the N/A and keep it running perfectly. Air filter mods *may* score a bit more "airflow", whether it is more of the heated underhood air you get is debatable.
A new set of spec spark plugs may get you the same power gain as some fancy filter, and the plugs will give improved efficiency and economy all through the range.