So back to memory.
Just checking prices, and on newegg.com, if you buy a G.Skill Ripjaws Z 32GB kit, it will be $1499 for a 4x 8GB, while the 8x 4GB costs $329.
These kits are both rated at 2133 MHz.
So I come back to my original question, what difference will it make for overclocking/stability that you use all 8 memory slots as compared to using 4?