DDR2 and DDR3 are different types of RAM.
Interested that you posted in graphics cards as well. Heres why:
For system memory, RAM that you can buy and replace on your motherboard, there isn't any performance difference. ddr3 is NOT faster at all significantly. Its a different type, you simply need to get the same type for compatibility. On laptops, ddr3 gives better battery life but not very significantly.
For graphics cards, this is called vRAM and the types are usually called gddr2 and gddr3. In this case, gddr3 is MUCH faster. A single model of card will be produced in both types, which is confusing, and for example an 8600 gddr3 is probably faster than a 9600 gddr2.
DDR3
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
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.
ddr3
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.
I think Dance Dance Revolution 4 was the best
DDR3 uses 1.5V; DDR2 uses either 1.8V or 2.5V
ddr3
There are 240 pins on DDR2 and DDR3.