Computer

I am currently using an ACER computer with a dual core AMD 64 bit processor and 1 GB of RAM. This is overkill for my server but it was cheap (~$400).

The main other consideration is hard drive bays. Since I am going to have a RAID 5 array plus a separate hard drive for the OS, I need space for 5 3.5" hard drives. The specs on this computer listed 6 3.5" drive bays, but it turns out that two of them are really for floppy drives. A hard disk will fit, but it is pretty tight. Instead I bought some brackets that converted one of the 5.25" bays to a 3.5" bay.

Unless the motherboard of your computer comes with RAID built in, you will need to buy a RAID card. I highly recommend 3ware cards because drivers for their cards are part of the Linux kernel starting with 2.6.19. Most RAID cards will fit into some kind of PCI slot. I am using a PCIe 4x RAID card which ensures that the bus will never be the limiting factor for data transfer. Make sure the motherboard has the slots you need. My motherboard has a 1x and a 16x slot. Bus rates are listed below. Wikipedia has some handy references for device bandwidths

SATA II 3000 Mbps
PCIe 4x 8000 Mbps
PCI 32 bit 33 MHz 1067 Mbps
Gigabit Ethernet 1000 Mbps

So you can see it is nice to have the PCIe interface, but it really bottlenecks at the Ethernet adapter unless you are operating the computer locally.