im currently unable to make up my mind on which would be a better investment; a pulley kit for my 2004 mazda rx-8 or a k&n cold air intake system should i buy the pulley or the intake?? which would give me more benefits

 

The intake will likely be the easier install but will only produce power in the higher rpm range. Frankly I've come to believe that they are more scam than go... especially for a street car.
The pulley kit on the other hand, will free up some existing power and perhaps help with the torque numbers too.
Neither will afford that much additional power, but I'm thinking the pulley kit might be the best "investment" overall. Remember to get the correct belt, and if the kit comes with one, get a spare before the install.