Here's a LITE-ON Dual layer DVDRW. If you don't need a DVDRW immediately, it's best to upgrade as late as possible so you get the newest technology available. I personally prefer two optical drives to allow for easier CD/DVD copying. Two drives also allow one to use the RW as a large capacity floppy, and the other as a disc reader so as to not disturb the one acting like the floppy.
Which case are you thinking about? So far I really like your system. I agree that it should be reasonably powerful and stable. Good luck w/ your build!