In your post you said you'd like to "boost" your HP. Just remember, some mods you do may make your car SLOWER! For instance the short ram intake that you see on eBay, it will suck in more air, but that air will be hotter (esp. in summer). The hotter that air is, the less HP your engine will make.
My suggestion, take off the oem air intake cap, intake hose, and intake bottle. Clean to "new" condition then re-install with a K&N drop in filter (replaces your oem white filter). Instead of re-installing the intake "bottle" put a cap in its place and tighten it so no air can leak by. With the filter and "bottle" removed, I THINK you will see 3 HP to the front wheels. (ie, my car will remain stock forever)
Our engines (7A-FE) make 115 pounds of torque at 2800RPM. That's a nice and low RPM to make full torque, messing around with the stock exhaust system, will greatly effect this. (ie, my exhaust system is also staying 100% stock)
In my opinion, the first modification to our cars (94-99 Celica with 1.8 engine) should be a lowering suspension (or springs) and 17" wheels and tires. 17"s will make us slightly slower, but cmon now... these cars are not race cars, they are comfortable, good looking, daily drivers.
If you want to go fast, look into a VR-4, (I would go on and on but, this isn't the right forum for that

).