If you want to just replace the drive mechanism that is within the case, go for the Pioneer DVR-106. It's available at NewEgg for about $120, with free shipping. That is the same drive that ships with brand new G5s, so it'll work.
Actually, you might want to just install it internally. You can sell the case and the CD-RW, and your old drive. Since the DVR-106 can do everything, you don't need any more optical drives unless you want faster CD burning than the 16x provided by the Superdrive, or a second optical drive for direct CD copying. If you sell all that stuff, you'll make up a good portion of the cost of the new drive.