Throw as much as you can at CPU/RAM for what you're doing. I don't believe the HD speeds are going to make a cost effective difference, and today's IDE/SATA drives are pretty good with a decent CPU behind them. I would definitely set up RAID, not sure which way would be best for you though - fault tolerance or throughput, or both if you don't mind using 4 disks (minimum for 0+1).
Once you've worked out your parts list send me a PM as I reckon I can compete with MSY, and my firm sponsored the SAU team at Autosalon...keep it in the family!