If an Apple employee told you to buy the Snow Leopard DVD and install it on a PowerMac G5 someone missed a briefing.
The last OS available for a G5 is Leopard 10.5.8, Snow Leopard will only install on an Intel based computer. Same goes for PowerBook G4's, no Snow Leopard for those either. My daughter upgraded her iPhone 4 to iOS6 via WiFi and couldn't sync to her PowerBook after it restarted. I had to migrate her account to my Intel iMac just for her to recover her photos, music and movies. I tried for several hours to downgrade her iPhone 4 to iOS5.1.1, the software downloads just fine but when it validates the image Apple denies the firmware installation, gives me a "firmware no longer authorized for this device" message.
Bad form Apple, not allowing you to downgrade to the previous version is outright blackmail, especially if they are no longer supporting the hardware that requires the older firmware.