Tough one. The slots on the G5 are 64-bit, 33MHz whereas the card only needs 32-bit but at 66MHz. Technically they are equivalent but I'm not sure I'd buy one on that basis. Could you phone Lacie and ask? Maybe they would let you try it and return it if it didn't work.
On the other hand, if it really does need 66MHz, the only Macs that will run it is the Smurf G3 and Yikes PCI G4. Seems unlikely to me. Its bound to work,