First off, everybody flaming just put a damn cork in it and get your heads out of your assholes. Sorry but this kind of immature reponses burns me.
Now to answer your question CelicaZ adding a nitrous oxide system is not gearter of a deal than possibly replacing/modifing your Intake
manifold to add in the system. Most systems i have seen offered by NOS can come with everything needed to perform the mod and usually NOS will have a specific system for each brand of car (i.e. Acura, Celica etc).
Now the long term effects on the engine are going to be resultant of the increse pressures and temperature in the combustion chamber putting higher mechanical stresses on the materials and moving parts (cracked housings, premature failure of pistons, connecting rods).
The most common problem i have seen is burning through of piston heads. In fact, my uncle had put a nitrous oxide on his camper to climb the huge hills (low metered doses) for going down in south america from canada and it worked well to 2 years until he made the drive back and found 3 pistons completly burned through out of 6. Had to come back on a flatbed.