RAM is labeled by its Memory Size (In MB) and clock speed (or bandwidth).
SDRAM (Synchronous Dynamic RAM) is labeled by its clock speed in megahertz
(MHz). For example, PC133 RAM runs at 133MHz. SDRAM is nearly
obsolete as nearly all motherboards have withdrawn support for SDRAM. It is
now superseded by the more efficient DDR RAM.
• 128MB SD-133 = 128MB PC133 RAM
DDR RAM can be labeled in two different ways. It can be labeled by
approximate bandwidth; as an example, 400MHz-effective DDR RAM has approximately
3.2GB/s of bandwidth, so it is commonly labeled as PC3200. It
can also be labeled by its effective clock speed; 400MHz effective DDR RAM
is also known as DDR-400. There is also DDR and DDR2 labeled as PC and
PC2.
• 256MB DDR-400 = 256MB PC 3200 RAM
• 256MB DDR2-400 = 256MB PC2 3200 RAM
DDR RAM has two versions: DDR (also DDRI) and DDR2 (or DDRII)
• DDR supports DDR-200, DDR-266, DDR-333, DDR-400 (mainstream)
and DDR-533 (rare)
• DDR2 supports DDR-400, DDR-533 (mainstream) and rare/expensive
DDR-667, DDR-800, DDR-1066