DIMM happens to be the size of the slot where you apply the SDRAM, DIMM is typically a 168-pin slot which is probably what you have if you have a most up to date or current motherboard. In older motherboards, you find SIMM slots which have only 72 pins. Basically, you are able to use much more memory today than like before. if you are using windows operating system you can go to the help section and enter the word DIMM in the search. This should get you started on a more clearer idea and as well lead you towards more information to your question.
240
168
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
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.
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.
SDRAM runs synchronized with the system clock
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.
184-pin DIMM
1 notch
168-pin SDRAM DIMM
dimm
Yes. For example the iMac uses PC2-5300 DDR2 SO-DIMM SDRAM memory.