Not a problem.. I bought an off the shelf DVD burner from Staples and slid it right in my old G4 400Mhz tower running OSX 10.3.9.. replacing the stock DVD-ROM drive.
Worst case scenario, the Mac will see the drive and read disks, but not recognize it as writable.. in which case, just download and install the Patch Burn software mentioned earlier and you'll be burnin' DVD's like a madman.
You might be able to just forgo the Patch Burn mod and use a third party application like Toast for all your disk burning needs, but you won't have the 'native' OSX support without Patch Burn mod if OSX doesn't support your particular drive. (like burning directly from the finder, Disk Utility, or iTunes)