I want to use Apple's software iDVD, version 3.  It seems to be only
compatible with Apple's internal SuperDrive DVD burner, and not with
any external drives (such as the generic one I just bought.)

My computer is a G4/400 machine with a 100Mhz bus speed and a built-in
DVD-RAM drive that is not a burner.

What are the specs for and where can I find an Apple DVD burning
SuperDrive that will directly install into my machine in place of the
drive in there, allowing me to use iDVD 3?

I installed an internal PIONEER DVD-RW DVR-107D burner in my G4 and it
works perfectly. Your G4 manuals shows how to change your old CD ROM
drive to another one.

So the Pioneer drive you installed is recognized by iDVD for DVD
burning?  A few articles I was reading this morning dating back about
a year were saying that only Apple's SuperDrive would be recognized by
iDVD3.

Apple's "Superdrive" IS a Pioneer DVR 107D, or in some cases a Sony
DW-U10A (in my G5's case). Either should work with iDVD, as long as it
is internally mounted. Apple usually does  things decently, but they
did a software lockout with external drives, probably to encourage
people to upgrade.

After some more searching yesterday I found a couple strangely named
files that when placed in my computer's root account directory,
changed the function of control-clicking the burn button in iDVD.

I can now control-click the burn button and bring up a dialog that
allows me to save the RIP as a disc image file.  I can then use Toast
to burn the disc image file.  That works for now.

Thank you very much for the clarification of the Pioneer DVR 107D
drive being the stock Apple Superdrive and one that will work with my
older 100 MHz bus G4.  I'll definitely consider adding this as it
would just be easier.