Once announced, GDDR4 will start working at the similar speeds of top GDDR3 chip but it will use less wats and will be easier and cheaper to produce. At least thats the plan. SO you can imagine that this memory should start around 800MHz efficiently 1600 MHz if not even higher.
However this memory will be different from GDDR3 so we dont think that each graphic chip out there supporting GDDR3 will be able to support GDDR4 memory. This memory will require some memory controller modifications, I guess so I don't expect that older cards will support this new and possible quite pricey memory.
The first chip that we are aware of to be using this memory will be next years Shader Model 4.0 Nvidia chip known as G80. µ
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