After a bit of research I bought a Pioneer 109(black) - for £50
Dual layer and very fast!
Apple reflashes the orignal drives to make them apple branded superdrives - so imovie etc can only see the apple drives. But there is a fix. Patchburn (www.patchburn.de) is a patch that allows you to use the off the shelf burners.
With the G5 tower however you do have to watch out for the drive mounting screws that raise the drives to a specific height as sony, pioneer etc are slightly different heights. Apple sell screw kits for £9 - bit up a mark up on that product!