It appears that I have the ATI Radeon 9600 card in my G5:
<<ATI Radeon 9600:
Chipset Model: ATY,RV351
Type: Display
Bus: AGP
Slot: SLOT-1
VRAM (Total): 128 MB
Vendor: ATI (0x1002)
Device ID: 0x4150
Revision ID: 0x0000
ROM Revision: 113-A58504-113>>
Can I run the 30" Cinema Display?