This guy built a uber-cheap 1.2TB RAID system for $1,600. I spent $2,500 for my 0.75TB system back in December, but I’ve been very happy with it. Extra money was spent on getting a decent UPS, a real hardware RAID controller, and a hot-swap case w/ a sizable power supply.
Looking back, I would have spent an extra $100 for an even better case, gotten the 3Ware 8506 card instead of the LSI MegaRAID 150, and gotten a motherboard w/ 64-bit PCI (bought one on-sale that didn’t have that, oops).
Anyway, I figure I’ll be good for at least another year or two, at which point I’ll probably swap out my array w/ 1-2TB drives.