Memory stick upgrade: How to select

Hello I want to add memory to my Gigabyte board with my processor I5 I know that I could have up to 32 gb on my board. Actiually, I got 8GB (2 X 4 gb). I went on Ebay, just to find out, their is a multitude of manufacturer, voltage etc. I want to add 2 + 8 gb modules How do I select the appropriate memory sticks. I don't play game, but I do use a lot of charts with a long historic...