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Many people assume that faster DDR2 memory will make a system much faster. This isn't true.

This isn't a problem most of you will have soon. However, because some laptops have the physical ability to insert >2GB memory, we feel like the issue should be addressed. Also, please understand that we haven't this scenario -- it's still theoretical.

DDR memory has been the standard for some time, but we will be slowly transitioning to DDR2 memory over the next 3-6 months. The Sager NP9860 (http://www.powernotebooks.com/specs/Sager/9860specs.php) already uses DDR2 533 memory. Original SDRAM (PC100/133) transferred data once per clock cycle. DDR memory (DDR333/400) transfers data twice per clock cycle. DDR2 memory follows that same path by transferring data 4 times per clock cycle.

Since DDR2 transfers effectively double the data per clock cycle, it should be twice as fast as DDR, right? Not quite. The reason is that DDR2 memory has a slower clock speed. DDR400 is clocked at 200 MHz, while DDR2/533 is clocked at 133 MHz. So yes, the DDR2 memory has a little bit more bandwidth (remember it is 4x faster than its clock speed 133x4=533), but not double because it is clocked slower. DDR2 memory is also slowed down because it generally has a 4 or 5 CAS latency vs. 2 or 2.5 latency for DDR. This means that any transfer with current DDR2 memory takes at least 2-3 clocks longer than it does with DDR. This slower response time negates the small bandwidth advantage, and DDR2/533 memory is generally identical in performance to DDR400 memory.
http://www.xbitlabs.com/articles/memory/display/ddr2_4.html
http://www.anandtech.com/cpuchipsets/showdoc.aspx?i=2088&p=17