Save the dosh, your existing setup is perfectly adequate for your needs, all you need to do is add a hardware RAID card, and an array of fast 10,000rpm or above drives. I don't know the performance diff between SATA II and SCSI but I'm pretty sure there's not much in it these days. SataII may even have the edge. Faster RPM doesn't guarantee better performance, access times and other factors are involved.
I wouldn't have thought database and email usage should mean very large file sizes?