if your only talking about your front discs (since the celica gt uses drums for the rear) then you can either get slotted or vented (you can get both but this weakens the rotor and makes it more susceptible to warping). slotted rotors will help you stop a little faster but it won't be significant. vented rotors just help your rotors cool faster to prevent warping under heavy, constant braking.
a better way is to just upgrade your brake pads. TRD
makes a set of brake pads that will help you stop faster..but they are about $110 for a set. 'green pads' also stop better over stock and are also much cheaper..you can buy them from www.tirerack.com
i beleive. try to stay away from racing brake pads because even though they stop better than any other types of pads they screech like crazy.
also, $300 is the quoted price on changing your rear drums into the celica gts disc brake...a pretty good conversion for not a very high price
hope this helps