DDR2 vs DDR3
DDR2 and DDR3 belong to the recent DDR SDRAM (double data rate synchronous dynamic random access memory) family of RAMs. Both these RAMs store data in similar type of DRAM arrays. The Initial member of this family was the DDR. DDR2 followed DDR. And, DDR3 is the member, which followed DDR2. Both these RAMs are not compatible with any other RAMs in the series. This means, in order to change your RAM from one type to the other (for example, upgrading from DDR2 to DDR3 RAM) you need to upgrade your whole motherboard.
What is DDR2?
DDR2 SDRAM stands for double data rate type two synchronous dynamic random access memory. It is the second member in the DDR family. However, DDR2 RAM is not backward compatible with DDR. That means you need two types of motherboards for DDR and DDR2 RAMs. It uses double pumping to transfer data on both the edges of the clock signal (much like DDR). DDR2 RAM provides a performance of four data transfers per clock cycle. Therefore DDR2 can provide a maximum transfer rate of 3200 MB/s (with a base clock speed of 100Mhz).
What is DDR3?
DDR3 SDRAM stands for double data rate type three synchronous dynamic random access memory, and it is the big brother of DDR2 SDRAM in the DDR family. However, DDR3 RAM is not backward compatible with DDR2 (or DDR for that matter) due to differences in signaling, voltages, etc. DDR3 RAM can transfer data at a rate that is twice as fast as DDR2 RAM. This means that DDR3 can provide higher bandwidth compared to DDR2. In fact, DDR3 can go up to a bandwidth of 6200 MB/s (with a base clock speed of 100Mhz).
What is the difference between DDR2 and DDR3?
DDR2 and DDR3 are improved RAMs belonging to the DDR family of RAMs. DDR2 RAM provides 4 data transfers/cycle, while DDR3 RAM provides 8 data transfers/cycle. That means if the base clock speed is 100Mhz, then DDR2 RAM will provide 3200 MB/s bandwidth, while DDR3 RAM will provide 6400 MB/s bandwidth. On the other hand, DDR3 RAM uses 1.5V per chip, whereas DDR2 RAM uses 1.8V per chip. DDR3 RAM supports 400-1600 Mhz I/O bus clock compared to only 200-800 Mhz clock in DDR2 RAM. So, in general DDR3 RAM is relatively faster and consumes less power.
However, choosing between a DDR2 RAM and DDR3 RAM is not always a decision based on performance. DDR3 RAMs cannot be plugged in to motherboards with DDR2 RAMs. That means, if you already have a DDR2 RAM, you have to upgrade the motherboard to get a DDR3 RAM (most of the times), and that could be costly, too. But in reality, both Intel and AMD have fully committed to DDR3 for the future, meaning that you will have to upgrade your motherboard at some point (if you still have a DDR2 RAM). Good news is that, the price of the DDR3 RAM was used to be much higher than DDR2 RAM, but it has come down slowly, and now both cost around the same price.