Spend another 10 minutes searching and reading the forums here and you will find "The Apple Core" forum. Lots of good information there. Mach2Music does mods to macminis. Another website, strictly dedicated to all things related is The Computer Audiophile.
Your current question is way too generic and open ended. All kinds of issues you need to get familiar with like; music playback software (Pure Music, Ammara, etc.), digital file formats, solid state drives, outboard hard drives, memory, digital interfaces (usb, firewire), power supplies, cables, dacs, etc.
