>>> I found this one: [...] Radeon R5 230 [...] PCIe x16 [...]
>>> Is this one useable for the G5?

>> Assuming you mean an AGP-based G5 machine,
>> I‘m not sure how you‘re going to insert a PCIe card ;-)

> I found this one: [...] Radeon-HD-6450 [...]
> Is this one a better choice?

PCIe again. Your G5 machine only has AGP and PCI(-X) slots, so this card won‘t work either.