168-pin SDRAM DIMM
The positions of two notches on a SDRAM DIMM identify the type of DIMM and the voltage requirement and also prevent the wrong type from being installed on a motherboard.
DDR SDRAM has one notch and uses 184 pins.
DIMM. Two types of DIMMs: a 168-pin SDRAM module (top) and a 184-pin DDR SDRAM module (bottom). Note that the SDRAM module has two notches (rectangular cuts or incisions) on the bottom edge, while the DDR1 SDRAM module has only one.
2.66" 189-pin SO-DIMM contains DDR SDRAM 2.66" 189-pin SO-DIMM contains DDR2 SDRAM 2.66" 145-pin SO-DIMM contains SDRAM 2.35" 40-pin SO-DIMMs (outdated) 130-pin SO-RIMM (conatains Rambus memory and has two notches).
Sdram has one notch. Depending upon the model of Sdram the number of pins it has can vary but a standard Sdram has 184 pins.
1 offset
168 pins are used on SDRAM DIMM. 184 pins are used on DDR DIMM. There are either 30 pins or 72 pins used on SIMM. 184 pins are used on RIMM.
168
240
one notch
You're computer will require a "DDR2 SDRAM DIMM". DDR2 SDRAM is the type of memory you are after, it is also a DIMM. Most modern memory are all DIMM's (Dual inline memory module), much older ram were SIMMS (single ingline memory module). DIMM refers more to the physical make up of the module (DIMMS have contacts on both sides of the module). So any DDR2 SDRAM that you buy IS a dimm. You may want to look at the wikipedia page about DDR2 SDRAM at http://en.wikipedia.org/wiki/DDR2_SDRAM