Q: I'm considering upgrading my RAM and have recently been looking at some at CompUSA, among other places. I know my computer uses DIMM, but I also see RAM with different amounts of pin numbers. How do I know how many pins my computer has?
A: Generally we look at the CPU type and model, and chipset version to figure out questions like this, but if you don't have that information handy it's easiest to just use a tool like the Crucial System Scanner: http://www.crucial.com/systemscanner/
In the upper left hand side of the report page the System Scanner will tell you if your PC is using DDR, DDR-2, RDRAM or SDRAM memory. The right hand pane notes if Dual Channel support is present, and if it is you'll want to buy pairs of identical RAM when you upgrade. When you shop for replacement memory, the type of memory is the basic info you'll need to know. From there you can decide on RAM speed and memory capacity easily enough.
Now, while you're right there are pin-count differences between each type of memory, stores and manufacturers reference the memory type (aka. DDR, DDR-2, RDRAM, SDRAM) and not the pin count as is done with CPUs. On one last note, the acronym DIMM (Dual Inline Memory Module), is the technical name for the circuit board that holds all them memory chips. All the above memory types are DIMMs, whereas RDRAM memory is a RIMM, which is basically a DIMM too.
Also check out PCSTATS Guide to RAM, memory and upgrading for more tips and advice.