Also see: What is the difference between DDR2 and DDR3 memory?
What memory type do you need? The type of memory your system supports dictates the type of memory you need for your upgrade. Check your owner's manual, under the specifications section, to determine what kind of memory your system uses.
Many DDR2 systems run their memory in "dual-channel" mode, which allows the system to move data to the memory at a rate up to twice as fast as it could with only one memory module. If your system supports dual-channel mode, install memory in matched pairs to best take advantage of dual channel and for optimal memory performance.
If the system that you are upgrading or building up uses DDR3, you have the latest in memory technology. DDR3 uses less power than a comparable DDR2 module. Also, DDR3 can operate at speeds much higher than DDR2, thereby transferring more data in a shorter period of time. Some DDR3 systems can even run memory in "triple-channel" mode which allows them to access three pieces of memory at the same time, increasing the amount of data that your memory can handle over a certain period of time even further.