Well, changing from RAID 5 to RAID 1 will always hit you in terms of performance.
But I'm running a SME server 6.0 on an old Asus P2B-LS board with a 400Mhz processor, 512 Mbytes RAM and two WD 60 Gb disks, without any real problems.
Mind you: Filetransfer of large files can be a tedious business. Otherwise - no probs.