I was recently out to buy a 4 GB RAM chip for my laptop, and all they asked me was whether I wanted DDR-3? When I specified that I had a 64 bit system, they said that it's not important, my question is why? Shouldn't my system's word processing capabilities be a factor? Isn't memory designed into registers the size of the system's word length???
No. Modern PC systems contain a memory interface chip or circuit that translates between the data bus width and the memory module width, the latter of which has been standardized by organizations such as JEDEC. This means that modules that use the same signaling and protocol are interchangeable regardless of the underlying architecture of the system.
The memory modules are standardized and have not been 32 bit in a while. even with a 32 bit processor there are layers of caching and on the dram side of the cache it doesnt matter what the processor prefers you want to read and write in multiples of that size so 64 bit is not a bad tradeoff. Likewise the (some, all, or latter) 32 bit processors adopted 64 bit busses anyway for similar reasons.
And if you look at the modules you either have a multiple of 8 chips (8 or 16) or a multiple of 9 (9 or 18) or 5,10 or 4,8
the chips often start 8 bits wide then over time can accomodate 16 bits wide for that speed. so you start off with 64 bits no ecc which is 64 bits per rank 8 parts per rank, one rank memories have one row of 8, two rank, 8 on one side 8 on the other. with ecc you get another 8 bits 72 bits, 9 parts per rank. for 16 bit wide memories 4 parts per rank no ecc and 5 with ecc.
But once you specify the connector/module type the choices become quite limited, how much ram and then depending on the brand/model you get whatever number of parts to implement that.
So for buying individual parts they need to know DDR, DDR2, DDR3 or DDR4, what speed, and density you want. The memory controller is specific to each and may not necessarily support anything but the one type, so a ddr2 may only support ddr2 and a ddr3 may only support ddr3, so you have to first start with that question, then move on into speed and with and density.