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.
By looking at the notch on the right
The position of the notch on the right identifies the voltage used by the module. The width of the notch <<<WRONG ANSWER! The width will be the same, but the notch can be in a different position. Please do us all a favor, if you don't know what you're talking about then don't post.
The notch on the right identifies the voltage used by the module.
All memory modules have an identification code to enable the correct dimm to be used. The code has never been designed to give any other information than the SDRAM on the DIMM. Only by going to the manufactures DATA specification sheets can voltage, dwell times, etc be found
SDRAM has 168 pins and two notches on the module.
168-pin SDRAM DIMM
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.
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
A DDR SDRAM module
In SDRAM, rising and falling edges are the opposite edges of a memory module through which data can be transferred. SDRAMs that are designed this way are called DDR (double data rate) SDRAMs.
While I'm sure a destructive lunatic could force a DDR module into an older slot, they are not designed to be physically compatible. DDR modules have only a single notch, while SDR modules and slots are keyed with two notches and ridges.
Kingston Technology 1 GB DDR2 SDRAM is great and has good ratings