i agree with all the comments so far, my opinions is this...
nos can be complicated, expensive and dangerous, it will take planning and alot of money to replace parts in your car to ones capable of running nos without troubles. if you have a newer car and/or a reputable garage or mechanic says your car is up to it then i would recommend because the results are sometimes quite breath taking.
i also agree with an early comment that its dangerous, nos on a street car can be lethal as im sure you can imagine. nos cars i driven in the past hit 90mph in second gear with only a few seconds of nos.
if you are looking for a higher top speed, new engine etc.
if you are looking for crazy accelloration then i suggest a a turbo charger or a super charge, these also would require new parts to your car but your looking at way more money in the bank this way:D
hope this helped (p.s i cant spell to save myself)