Ah, well, I doubt 8x will have a big performance impact on any AGP cards, ever. You see, the last of the top end cards are comming out soon. After that AGP will be a "lower" standard such as PCI is now.
Remember the TNT2 was available as AGP4x. But the Geforce didn't need it, and even the GeForce2 GTS only got a small gain from 4x. So small in fact that the BX chipset, the fastest available at the time, performed better even in games than available 4x chipsets.
I'm guessing the same thing will happen with the next generation of cards, which are being released soon. You'll probably see 8x giving them a barely noticable lead over 4x boards.
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