I've read such and found the documentation to never actually be written in the words you supplied. I have however found that documentation to write "1.5V supply required for 8X operation."

Please supply the link to your passage so we can tell.

In closing, there is no AGP 8X only card out there because... they would not sell.

The 8500 will work in 1, 2, 4 and 8X slots that it fits into.