Well... finally made it work.
It took hours of experiments, but at the end got the X5650 with 32GB@1333 (4x8GB, Kingston HyperX Fury) on my GA-EX58-UD5.
Here's what I did, for anyone else doing this:
1) BIOS version f13.
2) The disposition for my sticks is 3-4-5-6. Whenever I try to use 1-2, I...