You've gotten some good advice in previous responses. Just a few points to add.
Case needs to accomodate all drives with a couple of bays remaining for future addtions. One very good case is the Antec Sonata. It comes with an excellent 380 watt power supply and will accomodate 4 hard drives. Another good selection would be a CoolerMaster Cavalier case. Large and roomy, with plety of drive bays and well designed.
A quality power supply of about 400 watts or better is needed. Some good ones are Antec, Enermax, ThermalTake, CoolerMaster, and PC Power and Cooling (the best, but pricey).
Select a motherboard with at least 2 IDE channels and 4 SATA connectors. They are readily available and more common for Socket 754 and Socket 939 AMD 64 CPUs.
A moderately priced motherboard with a plethora of drive connections is the Gigabyte K8NS Pro. Will handle 4 SATA drives and 8 PATA (IDE) drives using on-board controllers and connectors. It is a Socket 754 board, and paired with an Athlon 64 3200+ (or better) and 1 GB (or more) of RAM provides excellent performance.
I use Western Digital and Seagate drives. Haven't seen any real difference in reliability, but Seagate SATA NCQ drives offer somewhat better performance and a longer warranty (5 yrs vs. 3 yrs).
For the absolute top SATA drive performance, use a Western Digital 74 GB Raptor drive as boot disk with primary applications. The Raptors are expensive but extremely fast.
Above all, don't scrimp on memory or power supply. Both need to be high quality to support your components. Crucial, Kingston, Corsair, and Mushkin all supply quality memory modules.
Dalton