According to This Extremetech.com Article, 8x AGP gives almost no performance boost over 4x AGP slots with current applications, and may only cause them to run with unneccessary instability.
Based on our initial testing, AGP 8X on today's architectures and with current applications doesn't appear to deliver any appreciable performance improvement for nVidia boards. The Radeon 9700's potential benefit from AGP 8X is still something of a question mark.
Should you get AGP 8X on your next motherboard? The answer for now: not if you have to pay for it. In addition, we recommend you stay far, far away from motherboards based on the SiS 648 chipset it's just too unstable.