All generations of the iMac G5 have dual layer writing support for DVD+R DL only (not DVD-R DL), with the exception of the very first generation (which is single layer only... I don't know an easy, non-technical way to tell apart Rev. A from the "Ambient Light Sensor" Rev. B iMac, although... I guess you could turn the lights out!
).
The easiest way to see which formats you support is to click on the apple menu in the upper left corner of the screen, then About This Mac, then "More Info." All supported formats are listed in the Disc Burning section of the System Profiler.