Will iDVD even run on a G3? (Mmm, guess it will according to the requirements, although it's going to be SLOW mastering a disc).
The answer to what I think you're asking is, if you're going to buy iLife '05, yes. iDVD 5 has the option of creating a disk image instead of burning a disc directly, and you can then transfer that disc image to another computer and burn it there. Pretty sure earlier versions of iDVD won't do that, though, so make sure you've got the latest, and you'll need a burning app that recognizes the image format used (.img disk image, whatever that is).
Two additional caveats: if you have no built-in DVD burner in the Mac, it's not going to want to install iDVD, and in fact, iDVD only ships on a DVD, so if you don't have at least a DVD-ROM, it'll be impossible to install it. If you do have a DVD-ROM but no DVD burner on the Mac you're probably going to have to do a bit of poking around on the install disc to get it on there.
Final question: With brand-new, top of the line DVD burners going for under $50, why wouldn't you just buy one to pop in the Mac if you're getting one anyway? It'd save time over transferring a several gigabyte file over a wireless network, at least.