There are four main types of RAM (in order from oldest to newest technologically); SD, DDR, DDR2, DDR3, and DDR4. They differ in the way they operate, and mainly in their operation speeds. All RAMs used in computers today have a synchronous interface meaning they wait for a clock signal before responding to clock signals.

From a more technical standpoint, RAM is a kind of computer memory that temporarily stores information, improving your system’s performance. Or in other words, RAM simply holds data for the processor. The processor also has its own memory known as the L2 cache, the cache lies between the processor and the RAM. The processor sends and retrieves data from this cache, but when this cache overflows data is sent to the RAM.

DDR2 SDRAM stands for double data rate 2 synchronous dynamic random access memories. DDR2 is twice as fast as DDR which means twice as much data is carried to the module for each clock cycle. Also due to the design improvements DDR2 consumes less power as compared to the DDR memory. DDR2 speeds range between 400 MHz (DDR2-400) and 800 MHz (DDR2-800). DDR2-400 transfers 3200 MB/s. DDR2-800 transfers 6400 MB/s.

DDR2 SDRAM stands for double data rate 2 synchronous dynamic random access memories. DDR2 is twice as fast as DDR which means twice as much data is carried to the module for each clock cycle. Also due to the design improvements DDR2 consumes less power as compared to the DDR memory. DDR2 speeds range between 400 MHz (DDR2-400) and 800 MHz (DDR2-800). DDR2-400 transfers 3200 MB/s. DDR2-800 transfers 6400 MB/s.

DDR3 SDRAM Stands for double data rate 3 synchronous dynamic random access memories. In theory DDR3 is supposed to act twice as fast as DDR2 memories. Thus DDR3 speeds range between 800 MHz (DDR3-800) and 1600 MHz (DDR3-1600). DDR3-800 transfers 6400 MB/s; DDR3-1600 transfers 12800 MB/s.

It depends on two factors-
a.    The amount of memory your computer hardware [Main board] can handle.
b.    The amount of memory your Microsoft® Windows® operating system (OS) can handle.
You can check your main board maximum capacity by using our “Product Compatibility Search” and for your operating system here is the list of the supported memory-

Windows 7 (32 Bit) Windows 7 (64 Bit) Windows Vista (32 bit)
Starter: 2GB Home Basic: 8GB Ultimate: 4 GB
All other Versions: 4GB Home Premium: 16GB Enterprise: 4 GB

Professional: 192GB Business: 4 GB

Enterprise: 192GB Home Premium: 4 GB

Ultimate: 192GB Home Basic: 4 GB

Starter: 1 GB
For more, please visit http://www.microsoft.com/whdc/system/platform/server/pae/paemem.mspx

No. DDR3 is not backward compatible with DDR2 and DDR. e.g. DDR3 has 1.5V power, DDR2 needs 1.8V and DDR needs 2.5V power and DDR3, DDR2 DIMMs come in a new 240-pin module. In terms of standard motherboard slots also the notch or the key for different memory modules is at different place for the three type of memory thus preventing any accidental insertion in memory slots which can damage the memory and or the motherboard.

Yes, it’s normal because when you install more than 4GB capacity, system resources (including ICH,AGP/PCI etc) will re-map memory address from upper memory area to lower area, that’s why BIOS always shows less than 4GB capacity.

Please follow the below steps-
1.    Please confirm the module is suitable for your computer. Cheek “Product Compatibility” section.
2.    Power off the computer and re-install the module again.
3.    Please try to change the order if more than one module is installed.
4.    Please try to install the TwinMOS module only/alone.

The Windows 32-bit protected-mode cache driver (Vcache) determines the maximum cache size based on the amount of RAM that is present when Windows starts. Vcache then reserves enough memory addresses to permit it to access a cache of the maximum size so that it can increase the cache to that size if needed. These addresses are allocated in a range of virtual addresses (3 to 4 gigabytes) known as the system arena.
On computers with large amounts of RAM, the maximum cache size can be large enough that Vcache consumes all of the addresses in the system arena, leaving no virtual memory addresses available for other functions, such as opening an MS-DOS prompt (creating a new virtual machine). To work around this problem, use the MaxFileCache setting in the System.ini file to reduce the maximum amount of memory that Vcache uses to 512 megabytes (524,288 KB) or less.
This issue would occur under Microsoft Windows 95/98/98SE/Me.
You may refer to the below URL for a solution.

http://support.microsoft.com/default.aspx?scid=kb;en-us;253912&Product=w98.