What are the checklist before buying a RAM?

There are 3 main things you'll need to know-
Memory type...very old systems used DDR memory, older systems used DDR2, most modern systems use DDR3, and the latest systems can use DDR4.

Memory speed...depending on which type your system uses, this could range anywhere from 100MHz to 2400MHz, or thereabouts.

Maximum amount per slot...all motherboards have a maximum amount of memory that can be used in each slot.

If you buy ram for a laptop or All-In-One it will always be SODIMM memory. Regular desktop systems use SDRAM memory.

Sites such as crucial.com have system scanners that will check your system and tell you all of the above. If you have a prebuilt system you can also go to the system makers website (search using system make and model number) for that info. If you have a custom built system you can find that info by doing a search using the motherboards make and model number.

1. Check what type of Ram your motherboard can support, eg Dimm
2. Check how much RAM you are expecting to use - probs 8GB. Make sure the CPU can support the amount they choose.
3. Speed - 1,333 MHz, 1,600MHz ect (higher is better)
4. Number of pins - mostly irrelevant

Knowing what RAM, your motherboard can support.

The RAM your computer supports & the max amount it supports.

The main things are "form factor" - the size & socket standard of the module (eg. DDR3 Desktop)
speed
and
capacity.

However, there are many other technical parameters and two apparently identical modules can have completely different compatibility.

The safest way is to use a "memory finder" tool on one of the memory sellers websites, which works from the make and model of the computer or motherboard to guarantee you get a compatible type.

You don't have to buy from them, but it gives you exact part numbers you can look up on other sellers sites.