No, a 256MB 9600SE is still much slower than a 128MB 9600.
Heck, even when it's just RAM speed (same interface) the faster RAM makes more of a difference in most games. For example, the standard 9600 Pro 128MB (DDR600) is often faster than the 9600 Pro 256MB (DDR400).
Cutting the interface in half has a similar affect to cutting the memory speed in half. Hence the 9500 Pro and 9700 Pro had the same chip, but the 9500 Pro had half the bandwidth, and couldn't keep up.
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