Both RAM size and RAM speed are important in their own rights. As far as RAM size is concerned, the basic rule is that you should have more RAM in your system than you can use in any scenario that you encounter on a daily basis. For most people, 16GB of RAM is plenty and for productivity-focused machines, usually, 32GB is recommended. As far as speed is concerned, it is less important than the RAM size in terms of critical system stability, but it is still important if you want a little bit more performance in applications like games.