I have Virtualbox running CentOS on my Mac, on which i have Oracle XE running (since there`s no Oracle XE for Mac). In addition to that, I have Eclipse, SQL Developer, Chrome, Firefox (both with gwt plugin) running. With only 4GB of RAM, this set up brought my Mac its knees.

I clearly needed to upgrade my RAM.

I notice that Mac has 4 memory slots. 2 are already occupied by default by 2 memory modules, each 2 GB. I bought another 2 modules, each 4 GB, tried to insert them to the vacant slots... and it works! Now i have 12 GB RAM.

RAM manufacturers may vary, but keep frequency (mine is 1333 MHz) the same.