Hexus has posted an article "Memory Bandwidth Limitations on Current Architectures"

It is, of course, widely known that even the 7.2Gb/s of bandwidth offered by the Ultra´s 230MHz DDR memory, the fastest and most expensive SDRAM currently available, is not sufficient to exploit the full potential of the Ultra´s core in 16-bit colour, let alone in 32-bit. In a reversal of the past approach to graphics card engineering, chips are now being designed to ensure that all available memory bandwidth is used - memory bandwidth has become the major bottleneck, rather than the core´s clock speed or number of texture units and pixel pipelines.

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