When used to describe RAM, the term DDR does not specify the actual speed of the memory. Instead, it refers to a "double(d) data rate" -- it's all relative to the speed of the motherboard's front side bus (FSB).

If your mobo's FSB is 100MHz and supports DDR memory, it supports 200MHz DDR RAM.

If your mobo's FSB is 133MHz and supports DDR memory, it supports 266MHz DDR RAM.

Both will be commonly referred to as DDR on hardware sites. It's confusing, but if you remember that DDR is a relative term, you'll be OK.