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.