Hi guys, this is what i read in everymac.com regarding the max RAM you can put on A1278. I hope it works. I will give it a try with 8GB:
"Apple officially supports a maximum of 4 GB of RAM. Originally, this model was only unofficially capable of stably supporting 6 GB of RAM, but as confirmed by site sponsor OWC, it is capable of supporting 8 GB of RAM if updated to Boot ROM Version MB51.007D.B03 and running MacOS X 10.6 "Snow Leopard" or higher"
Martin