The big issue with a second drive is that it takes up an IDE slot. Unless you're going SATA and have a lot of SATA ports available...
My fileserver currently has two IDE HDs and two IDE optical drives.

No more for me unless I get a SATA card for it and I hate to spend money on it given that it's almost 2 years old. I'm more likely to just get a new low-end machine from Staples or OfficeDepot or ... and move the data to the new one, using the old machine as a compile server or video transcoding server...