The 2000+ Toyota Celica's are an excellent platform for great car audio installations. Not only is the Celica an extremely eay car to work on, there are plenty of car audio products that will fit the stock locations perfectly.
The base of any good car audio setup generally begins with the headunit. The Celica's sotck stereo simply will not suffice when looking for a great setup. Changing the headunit should be the first step you take when upgrading your car audio setup. Any standard DIN or double DIN sized headunit should fit into the stock location. However you must remember that you will need an aftermarket wiring harness in order to successfully install your headunit as painless as possible.
High quality mids are also a must in any excellent car audio setup. The factory speakers simply will not be able to handle the power given by a modern high quality headunit. Not only are these speakers generally inexpensive they are also easy to install and in most cases are direct replacements for the factory units. Remember the higher quality the replacement you use the better overall sound quality and power your system will have.
Once you have installed a good aftermarket headunit and aftermarket speakers the next step is to add a high quality amplifier and subwoofer to your cars' setup. Any high quality amp should do just fine as long as it's accompanied by 1 or 2 high quality sibs. Most people agree that a good set of 10" subs fit great in the celica trunk and are easy to install given the adquate room of the Celica's trunk.
Custom enclosures are an excellent addition to any car and help set your car appart from others. Standard box type setup are also great and can be installed much cheaper than a custom setup. A good sound vibration insulator before you begin with you stereo installation will help remove the noisy rattling sound sometimes heard from the outside. Be sure to remove all interior panels and spray or adhere the sound insulator in just about every metal area possible.