Hi Mike
I am Dave, an Independent Advisor, I will help you with this . . .
8GB of RAM is plenty for Windows 10, the operating system will use approx. 2GB+, leaving you with 5.5GB approx. for your applications or games
You cannot choose between DDR3 and DDR4, your motherboard will only take either of those, those are not swappable, I will look into the type of RAM you can install on your PC for you . . .
What is the Make and exact Model No. of your PC - that will be printed on a sticker on the bottom casing
If that is a self-build, what is the Make and exact Model No. of your motherboard