Dirt Cheap High School Theater Sound?

Mainframe asks: "I am a student at a small (550 student) private school in Connecticut. I'm trying to bring theater here to a more professional level on a budget that is for all intents and purposes $0. I'm the sound tech, and at the moment we have a reluctantly two channel system with an amplifier and an 8 channel mixer. No CD player, no effects boxes, although everything is rack-mounted. Is there any software package that would make it simpler for me to run a show off of a computer? I need something that would allow me to play sound effects, mix in ambient sound, and perhaps even act as an effects processor for reverb, delay, etc through sound in/out. Right now I have an iBook running OSX doing effects, but it's hacked together using iTunes and a spectacular piece of software called Playthrough FX. OS is not an issue, since we have a robust IT department, so getting a PC with Windows, Linux, Unix, or a Mac with OS9 or X is not a problem."