Dave copy/pasted most of the right info about DDR.

With regards to nomenclature, the industry calls it "PC 1600" and "PC2100" because that's approximately how many Megabytes/second the memory can ("in theory") transfer per second. They are industry standards setup by an organization called JEDEC (you may have heard of them -- they're the ones that RAMBUS was a part of, and then left, and screwed them all by [secretly] submitting patents for SDRAM and DDRAM while getting them all to agree to them as standards, without telling them). PC2700 yeilds approximately 2.7Gigabytes of bandwidth per second ("in theory"). The memory industry put names to these because...well....because they needed some form of standard.

The graphics industry, on the other hand, changes the frequency of the memories they use all the time (generally going up). There is no "DDR standard" per se, in the same sense that there is in the consumer PC memory industry. They're not called "PC 7200" (or PC 10400) or whatever because they aren't industry standards. if this isn't enough let me know....but I gotta run to class now.

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A mathematician is a device for turning coffee into theorems -- P. Erdos