Notice how the red sides of the topmost box gets overlapped by the neighbors. The sides for that box are wasted pixels that never will show. I have minimized this problem with the option to turn on and off each side of the box... so a correct rendering of the above scenario would be to have a box without the sides, like this:
I can simply add two versions of the same image and reuse the texture memory and choose not to render the sides of the box. This saves valuable fill rate.
For anyone needing a frustum culling algorithm using OpenGL I can recommend this tutorial by Mark Morley.
Next up I got some serious bug fixing to do before implementing new features.