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A couple of reasons. The big one is, Rambus is not being included in Intel's future chipsets. The 850E will be the last desktop chipset from Intel that uses RDRAM. SiS showed a prototype PC1066 board with AGP 8X at the Intel Developer's Forum last fall, but that is the last anything was heard or seen about it.
The other reason is, AGP 8X on a current board is only useful as a marketing gimmick. There are no current games, or games in development, that will tap into the bandwidth available in 8X. By the time it is needed, your rig will be like a P3/1000 and a GF2 Ti is today.
BHD