If you're looking for more power swap the engine, turbo it or get a different car-- none of it cheap. Don't waste your money on minor upgrades of the B16B, it's so highly strung that without spending thousands you'll find it hard to make it any quicker. I have breathing mods, good for the noise but not worth the power gains imho as there's so little to be gained. Best way of making the car faster is to reduce weight - but that may not be feasible for you if it's your daily and you don't fancy driving around with no interior, ABS or aircon. If you want the car to feel quicker get a lighter flywheel, it will make it pick up the revs better and is cheap to buy. However the best investment for any car is a good set of tyres (and no, T1Rs are not an example of a good tyre).