Different size RAM in my MacBook Pro circa 2012?

My MacBook Pro 13" Unibody Mid 2012, 4GB RAM with Mavericks 10.9.1 has been glacially slow since installation. I have tried dozens of tricks recommended on the Internet to speed things up but the speed doesn't improve. Many have recommended upgrading memory and installing a SSD. My question is whether I can just buy one 8 GB chip and upgrade to 10 MG? Do both slots need to have the same size chip? Is there and downside to going to 10 GB with different sizes in the slots, assuming I can?