Which RAM memory cards should i buy?

That machine has an official limit of 32 GB of RAM, and can actually take 64 GB.

I would second the recommendation to buy RAM from Crucial or Other World Computing (MacSales). Use the tools on their sites to select the RAM that they say is compatible with your model.

Before adding RAM, you might want to check the Memory Pressure graph (in Activity Monitor) while you're running a normal workload. The graph is color-coded.

• Green = your Mac has enough RAM for what you're running
• Yellow = the amount of RAM is marginal for what you're running
• Red = the amount of RAM is insufficient for what you're running and is hurting performance

If all that you're seeing is Green, more RAM may not do much of anything to speed up your machine.

• If you currently have a mechanical hard drive, or a Fusion Drive, you may find that the best way to speed up that machine is with an external USB 3.0 SSD. On that model, a 1 TB Fusion Drive would include just a miserly 24 GB of SSD space – much less than the 128 GB Apple included with earlier models.

You'll find all the information about the RAM here: Install memory in an iMac - Apple Support. As to a vendor, OWC or Crucial are 2 great sources (I added Crucial RAM on my iMac.)

RAM will not increase the speed of your iMac. If you want to speed it up buy an external SDD and install that as your start up drive.

However on a 10 year old Mac you may want to consider replacing it. Yours is obsolete, slow, and will not run current versions of Mac OS.