I've been thinking about doing the same thing with my DVD's. I was looking at getting something like one of the items below.
http://www.newegg.com/Product/Product.aspx?Item=N82E16816701011
http://www.newegg.com/Product/Product.aspx?Item=N82E16816133001
http://www.newegg.com/Product/Product.aspx?Item=N82E16816702010

Reading the Serial Attached SCSI (SAS) wiki. You can connect SATA devices to the SAS controllers.

I found many sites detailing how to expand a raid 5 array in liunx. Here is just one example: http://scotgate.org/?p=107