From this
Seritek webpage..., it all depends on which model of G5 you have.
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The SeriTek/2SE4 is compatible with PCI-X bus architecture systems like Apple's Macintosh G5.
It is the only SATA controller card that offers full boot capability, allowing the fastest RAID configurations to include system operations. Four hot-swappable external SATA II ports enable individual drives or RAID arrays via built-in OS RAID 0/1 software. The card is
NOT compatible with PCI-Express slots like those found in Quad- or Dual-Core G5s or in the Intel-based Mac Pro. See FirmTek's SeriTek/SME4-E for support of these systems.