Because in the future a faster PCI bus could benefit all devices, such as maybe a new USB version that is faster than our current PCI bus can support. The new PCI express wouldn't be focused around just video cards. This would also benefit IDE controller cards, with the raid controllers getting faster, it will take more speed. It's all about thinking for the future. Sure, it may not have much advantage now, but it will soon. Heck, right now, the difference between 4x and 8x AGP is quite small. Just give it time.
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