I am also interested in this question; I am considering something like this:

http://www.addonics.com/products/io/adsaide44.asp

as SATA discs are much cheaper, and PATA is not going to get easier to find in the future. Plus I want to go to SSD, and again, the SATA variants are significantly cheaper.

I'm thinking I can fit this by mounting only a single disc sideways on the HDD tray.