The flashing question mark folder means that it's not detecting any bootable device. You could try finding a dmg of MacOS X Leopard (not Snow Leopard as that's Intel Only) and burning it to a Dual Layer DVD disc. Then you should be able to boot and install from that by holding down the option key while starting up (gives you a list of available bootable drives). I had two PowerMac G5s a while back and they both were pretty stubborn when it came to bootable USB, but bootable CDs and DVDs were no problem.
Regarding the Invalid Memory Access error, have you tried swapping out the RAM? It may be that you have a bad stick of RAM. Depending on the model you have, you've either got DDR or DDR2 RAM which are fairly common and cheap when bought second hand (there's at least a pair rattling around in every man-drawer out there). Again, depending on your model, you could install up to 16GB of ram!
Also, If you're feeling particularly adventurous, you could install a flavour of Linux instead of OS X.
Debian and Ubuntu spring to mind:
https://www.debian.org/ports/powerpc/inst/pmac
https://wiki.ubuntu.com/PowerPCDownloads