Both cards are dual head capable (indeed, the Radeons are even dual link DVI!).

Check it out here: http://www.xlr8yourmac.com/Graphics/FX5200_Mods_for_G4/FX5200_G4mods.html

The mod is pretty radical, and in some cases may require the use of a Dremel or hack saw. I got the 9600 to fit without hacking any bits off, but the FX5200 I tried first did need to have some surgery. This is because the location of the ADC power connector on the G5 is different than on the G4...

Peace,
Drew
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Power to the PowerPC!