Dont put a nitrous kit on a stock motor, youll run the risk of blowing it up because a stock motor cant handle nitrous as well as a modified motor. Maybe the next best thing you can do and its relativly cheap is a set of pulleys. Should cost you about 200, and you can probly expect about 8hp from it. A throttle body would be another good upgrade, that will run u about 400 and you should notice about the same as the pulleys. That puts you about to your limit for now, with the last 200 id look at maybe doing an MSD 6AL igniton or if you have a M6 get a short throw shifter. The MSD will give you quicker throttle response but hardly if any power gain, and of course you wont gain anything from the shifter, but those are both worth while upgrades, id do the shifter before id do the ignition. Or you could get a set of gears 3:73 if you have an A4 or 4:10s if you have an M6, that will eat up most of your budget but you will def notice the differance. Dont start fooling with nitrous and blowers until you have made the internals of your engine stronger (forged rods, cam,forged pistons, ETC)so you can fully optimize it and not blow your motor. Good luck.