You should open the case, or check the "About this Mac" dialog in the Apple menu: there's a fair chance your G5 has two processors, as in two complete packages. All series of G5s were available in dual-socket variants; the very last series of G5s was available with a dual-core processor, resulting in up to four cores in total.
Wikipedia has the details:
I’ve got a 2005 PowerMac G5, Dual 2.3 GHz with a stock Nvidia GeForce 6600 running OSX 10.4 Tiger
AFAIR that's a plain 6600, right?
Would adding a second GeForce 6600 from another PowerMac work or even be useful?
I believe the GeForce 6600 supports only a single Dual-Link DVI output (while it has 2x DVI ports on the back) so it should support ...