Your RAM is fairly close to maximum, though some have upgraded higher. I would not unless I found an application that was PowerPC Mac compatible that actually could use more RAM. If you do, realize that the operating system may not be able to address more RAM. You can in theory get up to 16 GB of RAM. That said, be sure to get RAM for Quad Core PowerMac G5, and not just any 533 MHz PC2-4200 DDR2 SDRAM, as Apple timing is very exact, and more so than the spec would seem to suggest. Do not get ValueRAM. Both http://www.macsales.com/ and http://www.crucial.com/ sell RAM worldwide specifically for the Quad Core PowerMac G5. Given the fact no Intel only Mac applications will work with your Mac, the cost/benefit ratio of souping up your machine may or may not fly.
If you do end up getting a newer machine, see this tip: