Well, if you want 300hp, you're definitely gonna pay for 300hp.
"Flashing the PCM" is what they call it when you reprogram the computer(Powertrain Control Module) in your car. This allows you to optimize timing, fuel usage, fuel mix, turn off the 2nd O2 sensor, adjust radiator fan settings, etc...
You'll have to either take it to someone who can do it for you, or buy the already programmed PCM from INTENSE. If you buy the PCM from INTENSE it costs $100, plus a $50 core charge that you get back when you mail them your old PCM.
The smaller supercharger pulley turns your supercharger faster, increasing boost, thus HP.
Cold air and fenderwell intakes bring cooler, denser air into your engine, giving the combustion process more oxygen, thus more HP. It also supplies the air that you are going to need to keep you SC fed, if you get the pulley.
You are also going to need a 180* or 160* thermostat, to keep your engine temps down.
If you are serious about that 300hp wish, check out the Stage 2 performance package from INTENSE, http://www.intense-racing.com/
use the menu on the right to look at their products.