xs engineering makes a nice turbo, and the blitz supercharger is also very nice (also less problems than a turbo). both retail around $3,500.
gotta agree with breakinankles on the NOS, its relatively cheap (except that you have to keep buying it over and over again) but its some of the most hp you can get for the buck.
the trial header (about $400) can also give you about an extra 11 hp with even more if you add an aftermarket exhaust.
a shorshifter doesnt add any power, but it definately makes your shifts quicker...and thats just as good as horses and torque if you ask me.
XS kit runs for 4500. Although they are trying to get a group buy for like 4300 or 4400 shipped.
You need to know what kind of celica you have before we can suggest what turbos. Year and model.