Memory Upgrade on a Mac Book Pro 13''

I have an early 2011 13 inch Mac Book Pro and I want to upgrade its memory. It came with a 4GB 1333Mhz DDR3 Ram. I found this kit online: http://www.amazon.com/Kingston-Technology-Notebook-KTA-MB1066K2-8G/dp/B001PS9UKW/ref=pd_cp_pc_3

However, the description of the item says that its 1066Mhz. I'm not really a tech guy and have no idea what this means but it seems like buying this and installing it on my Mac would be a downgrade as 1333Mhz is greater than 1066Mhz. Any thoughts? Would it actually be a downgrade? Is there an 8gb Mac Book Pro 13'' kit that is an upgrade in every aspect?