while u r maintaining a server, CPU/RAM wont count that much, but the Hard Drive efficency will definately be.
you put in 3000MHZ CPU + cheaper H/D = server failed. how much the CPU/RAM constitues in server responces? may be lesss than 10%
and if to see H/D's functionality = 100%
400MHz is good to go (uinles u r giving facility of database drives websites).
get very good hard drives (SCSI or S-ATA with good buffers/Spindle Rotation)