I wondered... all the new cards from ATI state as requirement "Power Mac G5" (the X800 for exmple), but I wondered why? Isn't AGP basically backwards compatible so that an AGP 8x card can work in an AGP 4x slot?
Most AGP 8x cards will work down to 4x, but they run at a voltage of 1.5. If the previous systems used AGP cards at a voltage of 3v, they won't work. You can actually do damage to the card and/or motherboard if you do this. I mistakenly installed an older AGP card to a PC motherboard that supported AGP 8x. Thankfully nothiing bad happened. The motherboard suported the 1.5v AGP and the card was a 3v AGP card.