they are different ram generations. DDR2 is the older and slower version of DDR3. DDR2 is about 800 MHz max. and DDR3 is about 1333 MHz minimum
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DDR3
DDR2 is slower than DDR3 due to their difference in bandwidth. DDR3 has a higher bandwidth hence can transfer data at higher speeds than DDR2.
It is not very noticeable if you upgrade your memory on your computer.
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DDR3 uses lower voltage
The difference between the memory of the DDR2 RAM and the DDR3 RAM are the capacity that they can hold the DDR3 RAM is more powerful than the DDR2 RAM.
DDR3 uses 1.5V; DDR2 uses either 1.8V or 2.5V
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There are 240 pins on DDR2 and DDR3.
DDR3 is twice as fast as DDR2.It's currently the best type of memory generally available.
DDR3 uses lower voltage