Are these components compatible?

Okay another thing. Is it possible to get 1 4gb stick right now, for example this:http://www.newegg.com/Product/Product.aspx?Item=N82E16820231310 and then get another stick exactly like it later, and have 8 gb total? Basically is it possible to use 2 of these as 8gb ram total?
 
Thank you you've been extremely helpful, one last question. Is the motherboard I linked powerful enough to run the CPU at full power? Because my last PC had an issue where the motherboard wasn't strong enough to run my CPU at full speed.
 
Did you watch the video I linked?