Sorry, I was a little rushed when I posted before! I'll see if I can go into a little more detail. Since you mentioned machining heads, I thought perhaps you were a bit more experienced... that's not a simple procedure at all! (plus, you only have one head... do you come from a V8 background perhaps?)
Anyway, to further explain the I/H/E stuff:
The stock plastic intake junk (tubing running from the front of the car near the headlight to the intake manifold toward the back of the engine) can be removed and replaced with an aftermarket piece. There are two types of aftermarket intakes commonly used. The first is the easiest, and it's called a Short Ram intake. It's basically a short length of smoothly bent piping (called mandrel bent piping) that attaches to the throttle body. On the other end is a cone shaped air filter. This type of intake is SIMPLE to install, requires no modification, and increases power a bit. It allows the engine to suck in air more directly. More air = more power. The second type is the Cold Air Intake (commonly referred to as CAI) This intake attaches to the throttle body just like the short ram, but it passes through the engine bay into the fender well (basically in front of the front wheel, behind the plastic water shield). This sort of intake does the same thing as the short ram, but rather than sucking in hot engine bay air, it sucks in colder air from outside. Colder air is denser, and has more oxygen. Denser air creates more power.
The cold air intakes generally cost a bit more, but not MUCH more. I have one on my Accord, and I like it. Good brands for either type are AEM and Injen. Be aware that the CAI can suck in water if you go into a large puddle! That will destroy your engine! Many people stay away from those types because of that. AEM does make a "bypass valve" that can prevent that from happening... but that's getting ahead of things.
You can find cheap intakes on Ebay. They don't HAVE to be the expensive brands... cheap is never better, but if you're not looking to squeeze out every possible horsepower, you can go cheap. As far as the intake pipe is concerned quality wise, a pipe is a pipe!
You can also replace your header (exhaust manifold). That is on the front of your engine... 4 pipes coming from the motor that are merged to form your single exhaust pipe. The stock pieces are often rough cast iron pieces, or crudely formed steel (if you're lucky). Aftermarket pieces with mandrel bent piping can make a HUGE difference! Header pricing can vary greatly. You can get a cheapie for under $100 US... or you can get a work of art for $1200 US. The cheapies might give you 3-4hp, the work of art can give up to 20hp. If you want something inexpensive that's decent, you can get an OBX header on Ebay for under $200 US (sorry for not converting...)
As for exhaust, there are many things you can do. I went to a muffler shop and had them weld on a custom setup for me. I don't have mandrel bent piping (meaning I have narrow points where the bends are) and I have a cheap muffler. It's not very quiet, but it sounds good. There ARE mufflers out there that are very free flowing (airflow is key) that don't make a lot of noise. Look into Apex'i and Magnaflow for quiet performance mufflers. If you get a muffler shop to bend your piping, you can add a quality muffler for performance AND quiet... an "axleback" system is what you'd shop for. A full exhaust system is often called a "catback" or "aftercat" system... it goes from the catalytic converter back. A good size for your car would probably be 2". MAYBE 2.25", but that could make you lose too much low end.
The purpose of the I/H/E combo is to get greater airflow into and out of the engine. The engine is essentially a large air pump. The easier it is to get air to flow, the more power you'll make. The key is to make sure air flows at the proper velocity. By upgrading all 3 parts, you eliminate some major stock bottlenecks (if you have the intake and header, but no exhaust, the extra air you suck in won't be able to get out properly). You should be able to gain 15-20 horsepower with decent parts... give or take.
If you're looking for low end, I would recommend getting a short ram intake, a 4-2-1 header (4 pipes merging into 2, then those 2 into one), and a 2" diameter exhaust. That should add some grunt to your car, and you'll gain the high end as well!
