I would have to agree with the rest of them... its a ridiculous amount of money to swap the motors.

As for the other question... the 1.8L out of a Celica GTS.
I happen to own a 2000 GTS as well as my 99 GS and you would be regressing to put that motor in your car. It might be a LITTLE faster at first, but the options you have to upgrade the celica motor are pricey and hard to find, its just too new yet. You should read some posts on here and talk to some of these guys, youd be surprised at what our "lil neon motor" can do :)