"DDR" is actually Double Data Rate SDRAM. "SDRAM" is actually Single Data Rate SDRAM. So the DDR stuff is twice as fast. But if you're looking at a GeForce2 MX card of any kind, they only support DDR at 64-bit or SDRAM at 128-bit. 128-bit is twice as wide as 64-bit, and the chip preferes 128-bit, so that even though the bandwidth is the same, the SDR card performs better.
Other cards (besided the GeForce2 MX) don't have this limitation, DDR is always faster for cards like the GeForce 256, Radeon, etc.
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