You don't need to purchase the drive from apple, as it's available almost anywhere. The original imac superdrive is the Pioneer A04, and the newest one is the A05. The first drive burns CD's at 8x, and DVD's at 2x. The new Drive I believe burns at 16x, and I know it burns DVD's at 4x. All you have to do is swap the drives, and you should be good to go with iDVD.
Anim8r