| Memory (RAM) |
The computer systems physical memory which is referred to as main memory or RAM (random-access memory,) which are chips capable of holding data. This is where the computer system holds current program and data that is in use. The term “memory” is somewhat ambiguous; it can refer to many different parts of the PC because there are different kinds of memory a computer uses. However, when used by itself, “memory” usually refers to the main system memory, which holds the instructions that CPU executes and the data those instructions work with.
http://en.wikipedia.org/wiki/RAM
Size:
The physical memory size is represented in megabytes (MB) or gigabytes (GB), where one GB equals 1000 MB. The more amount of RAM is one of the most efficient and cost-effective ways of boosting performance. Today’s computer systems running Windows XP or Vista should utilize at least 512MB of memory for adequate performance.
http://www.computermemoryupgrade.net/why-more-ram-is-better.html
DDR2DDR2 is a SDRAM variant that replaces DDR-SDRAM, offering greater bandwidth and density in a smaller package along with a reduction in power consumption. It also offers new features that compliment higher clock rate CPUs and 64 bit architecture.
http://en.wikipedia.org/wiki/DDR2
DDR-xxx or DDR2-xxx:
Where xxx denotes effective clockspeed.
PC-xxxx or PC2-xxxx
Where xxxx denotes theoretical bandwidth.