RAM - Improving performance on MacBook 13Inch Early 2011 What is better option to improve RAM performance eon MacBook 13Inch Early 2011:

Removing the original Apple RAM Memory slots of 2GB each (2 slots - each 2GB) and inserting brand new 2 slots of 4GB each 
OR
adding 2 new slots of 2GB each ?

And, what is the best Non-Apple brand for compatible Memory Slots ?

thank you in advance

Finn
I have the MacBook 13" Late 2011 and this is what I upgraded to and it works perfectly fine and so much faster!

This set is specifically made to work in Apple MacBooks.

Crucial 8GB (4GBx2) - Amazon