Man I love ifixit.com. They’ve got a great looking (and easy to navigate) site for buying Mac parts along with dozens of custom made instruction manuals for many Mac computers.
But today they’ve stepped it up — showing us how to hack the Mini (by removing the optical drive) in order to fit a second hard drive, allowing up to 1TB of storage (with 2x500gb sata drives).

It involves a little soldiering, but looks pretty easy if you know what you’re doing: take a look at ifixit’s Mac Mini 1TB Installation Guide and upgrade kit ($249 for everything you need, minus soldiering tools).