Memory Modules

Common Memory

Form Factors:

DIMM

long strips. Common in larger desktop and servers.

SO-DIMM

short strips. Common in most laptops and tiny desktops (Mac Mini).

Types:

DDR (or DDR1) -- You can use a higher number than called for, but not a smaller one.

Module names: PC2100, PC2700, PC3200

184 pin DIMM or 200 pin SO-DIMM

DDR2 -- You can use a higher number than called for, but not a smaller one.

Module names: PC2-3200, PC2-4200, PC2-5300, PC2-6400, PC2-8500

240 pin DIMM or 200 pin SO-DIMM

DDR3 --

Module names: PC3-6400, PC3-8500, PC3-10666, PC3-12800, PC3-14900

240 pin DIMM or 204-pin SO-DIMM

Notes:

Beware, DRR and DDR2 chips in SO-DIMM form factor can be hard to tell apart. They both have 200 pins and one notch. The notch in both forms is in a very similar spot. A DDR SO-DIMM almost seems to fit a DDR2 SO-DIMM socket.

DDR2 doubles bus speed, but also double latency. This can make naive comparisons to DDR speed difficult. DDR3 does not have this latency problem.