Well just about everyone you speak to will tell you something else. The cheapest solution is going with a nitrous kit. With a 300hp car and a 100 shot of nitrous you will hit your target power easily and very cheaply.
There really is no "best" solution to making hp, its really a matter of personal preference. Nitrous if great for making cheap horsepower but the downside is you need to refill the bottle all of the time. At $40 a bottle you wind up spending alot of money on that stuff over the years, but the initial investment is quite cheap.
Superchargers and Turbo setups are really nice, but are much more expensive. The great thing about this type of setup is that although the initial cost is high you have power on tap. No bottle filling.
The third solution would be good ol' horsepower. (aka "naturally aspirated") Many people like this type of setup because its "simple", meaning there are no blower/turbo tubes, intercoolers, extra belts, etc. Problem with type of setup is that you need to run a pretty radical tune to get the hp you want. Things such as larger heads, cams, etc are great for the track, or an occasional street/strip car, but not for an everyday driver. Big cams suck down the fuel quick!
My personal preference is the supercharger route. Turbo's make great horsepower but cost a bit more and are much more complicated in the install process. Although you need to drop $3000-$4000 on the initial cost a supercharger will make big horsepower, and wont kill you in gas costs when driving on the highway if you get a centrifugal model. Dont get me wrong, any real horsepower increase will use more fuel, but a centrifugal supercharger doesnt spin very fast at lower rpms (highway cruising) so you save wear and tear on the car, and the wallet LOL