I have exactly the same problem. I put 32Gb RAM into my Gigabyte GA-Z68MA-D2H-B3 board (with Gigabyte Radeon HD6850 graphics), did the install step by step, and only 4Gb of RAM shows up.
This is of course after installing the OS around 6x as the instructions for the SandyBridge patch aren't right and I was getting freezes after upgrading from 10.6.3 to 10.6.8. RAM is Patriot G2 (PC3-10666 1333MHz, 4x8Gb Kit). Had to try various steps out of order.
I'm not sure if its BIOS settings, but my board recognized the RAM off the bat.
All suggestions are very much welcome...