Sorry, jobi, but you will have to hunt down the solution yourself; I have no experience with G5 desktops.
The PCI bus was changed on the G5s to either PCI-X or PCIe; there are early PCI-X models that accepts some standard PCI cards. When choosing PCI cards take care that they are dual-voltage (will work in both 3.3V and 5.0V slots).
The later ones have PCIe (often called PCI-E) and are completely incompatible. PCI & PCI-X are parallel ports and PCIe is serial.
Download a copy of MacTracker (free, and Canadian to boot) to get compatibility info for Mac models.
More help can be found at Wikipedia
PCI-X - Wikipedia, the free encyclopedia
and XLR8YourMac
Accelerate Your Macintosh! News Page - 12/21/09
My investment in PCI cards for my G4 prevented me from considering an upgrade to the G5 desktops; the early ones were not enough of a jump in performance and the later ones are incompatible.