The computer is a glorified print server - NAS - Fax machine So, Ive been looking at 100 CPU's like the Intel i3-530 and the Core 2 Duo processors..
My NAS box at home runs windows 2003, with software raid 5 across 3 300gb disks. It has an old 800mhz PIII processor and 128mb of ram, and I can still easily saturate the 100mbps connection during file transfers! (well, during downloads... uploads run about 50%) The machine is also a backup domain controller and print server, runs just fine.
So, the cheapest processor you can find should work very nicely for NAS.