Well, internal is the way to go if you want to use iDVD. It's a no go with external burners.
Buuuut.....I would wait. Very soon Pioneer is shipping the DVR-107 (or A07) drive. 8x DVD-R burning! I'd get that, or at least wait untill it is released because you know the price of everything else will drop.
If you need it now buy a DVR-106, it's the same drive as Apple uses in the G5. It's plug and play supported in OS 10.2.8 and later (or maybe 10.2.6, I forget)
Here is a blurb about the 107
Blah Blah Blah... It's not supported yet..... It will be soon. I bet Apple uses 'em as the "New Superdrive" soon...