How much power can I add?

Generally, the amount of power you can add using nitrous depends on your engine size and current power output, which I can measure using an on-road dyno and provide you with a graph of your stock engine and plus nitrous.
On engine sizes up to a 1000cc, 25bhp in a single shot boost will give you a huge buzz. If you want to go higher, a nitrous controller will be required to introduce the nitrous gradually and avoid any damage. A controller also looks very cool on you dash and gives the 'pilot' the ability to change the boost settings (launch control), which will give much better 'off the lights' traction. See here for more controller info: Minimax Nitrous Controller.
Most engines up to 2000cc will take 75bhp extra (or more) using a controller. Larger capacity engines could go up to 150bhp in a single shot and still more with a controller. Theoretically, there is no limit to the amount of boost you can add if modifications are made to the engine to cope with the extra power. Highpower systems are used on top-fuel dragsters with over 1000bhp boost!
It is recommended that you add no more than 40% of your stock engines power without using a controller (as a basic rule) , but power gains of 100% are acheivable. A 25hp increase is a significant improvement in power and can be added safely to practically any car!

There are other factors to consider when looking at power gains over 100hp. Some inlet manifold designs make it necessary to have direct port nitrous injection to get good distribution to all cylinders and the standard fuel pump may need replacing to meet the demand for extra fuel. The ignition system usually requires some upgrade too for larger power increases. Read here for general nitrous do's and don'ts.

The Street-blaster 150i is the highest quality Nitrous system in the world. It is also the simplest system to install and is the only system designed to fit a standard street car without any additional major modifications. The system is supplied with all the parts needed including our superior Pulsoid solenoid valves, a 5lb alloy Nitrous cylinder (with brackets), a single Cross-Fire Injector, all pipes fittings and switches, plus simple and easy to follow instructions for the simplest quickest installation possible. This system has the potential to increase the power of your car by an extra 100+ horsepower, it is initially supplied with metering jets to deliver 25bhp extra. Bigger jets are available only after test results (Static test) have been reported to us for appraisal. Street-blaster 100i is only suitable for injected normally aspirated & Turbo cars.
Kits are also available for carburetter cars, bikes, higher power increases (300bhp+) and with multi-port injection.