George Ou over at ZDNet.com has a blog post about adding a good chunk of storage to his computer. All that is needed to duplicate this is 3 free drive bays, 5 free SATA ports, and a power supply that can handle the extra 150 watts while the drives spin up.
Fast forward 15 years later, and I just bought 2.5 terabytes of hot-swappable SATA-300 storage for just under $730.
I just bought 9TB (6×1.5TB SATA) at Microcenter for about $850.