What about something like this?
NCIX.com - Buy Intel BOXD945GCLF2 MINI-ITX DDR2 945GC Dual Core Atom 1PCI Motherboard Retail - BOXD945GCLF2 In Canada. NCIX.com - Buy Chenbro ES34069 MINI-ITX Home SERVER/NAS Chassis - Includes External PSU - ES34069-BK-120 In Canada.
Though you'd need to get a PCI RAID card, but otherwise its cool running, has enough power and is fairly affordable. There's probably cheaper cases too that you could use but that was just one that's kinda made for this purpose.