The Superdrive as a BTO on a G4 Tower is a steal
It's only £164 to have the superdrive instead of the CDRW (which can't be worth more than £60 or £70 at a push).
If you shop around for a superdrive the same as the one the G4s come with you're looking at spending over £250 and you wouldn't get iDVD with it either, saying that, a firewire case plus the bare internal drive for about £350 as a "roll you're own" DVD-R drive could be pretty good if it works fine with iDVD and iTunes for burning.
I'm definately looking at getting a superdrive with whatever model G4 I can afford next year, even if it means going for the bottom of the range G4 just so I can afford it.
Right now, seeing as I have no need for a modem, a little playing around with the BTO options on the applestore can get me an 800Mhz G4 with superdrive for about £1500. It wouldn't surprise me if the iMac with superdrive isn't the most cost effective mac in the whole range given the LCD and 800Mhz G4. It's certainly worth the extra over the model below it if an iMac is all you need.