That link that Wayne posted makes a lot of good points. It plays both sides of the coin, however, and doesn't give you a solid answer as to whether or not the 8x card you're looking at will work for sure.
When AGP 8x first came out, the cards that soon followed were able to run at either 0.8v or 1.5v, staying compatible with older boards. However, some 8x cards later on didn't bother with extensive testing or backwards compatibility. So although the AGP 3.0 spec states that it must be able to run in AGP 4x mode, it doesn't force manufacturers to include 1.5v compatibility, which is what most older AGP 4x boards support.
AGP 8x wasn't around nearly as long as 4x. PCI-Express came out shortly after and became the mainstream choice replacing AGP. So in a nutshell, you're going to have to research a card to see if it has FULL support for 1.5v. Check the card's manual (can usually download it online), call the manufacturer, get a guarantee from the vendor you're purchasing it from, etc. Cover as many bases as you can!
You can't just go by the model either. One brand of the nVidia 6600 might not have what you're looking for, but another might for example...
Good luck!
~cdogg
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