[Japo]

No, by now the problem has been perfectly identified. There has been no problem since I cut back to 8 GB. I was just dumb enough to plan an upgrade without looking at the motherboard manual--and lucky enough that, even though it was designed for 2 GB sticks at most, 4 of them at most, it works with 4 GB sticks too (as long as you don't plug more than 2, of course).

As I said the motherboard is a Dell version of a particular Intel one, but I have no way of knowing whether it's been modified only to print Dell's name, or further. But the memory's been working without trouble for a long time now, and it was always well paired (sticks bought together)--not that it's necessary, the computer can also work in single or mixed channel without trouble. Of course I didn't buy it from Dell, they expect you to pay triple, but by now it's clear it works with my motherboard. Though now I understand why Dell's website offered no sticks bigger than 2 GB for my computer...

Also Dell's BIOS doesn't allow you to tweak anything, no overclocking, no manual memory timings, nothing. It's possible that the motherboard is so identical to Intel's non OEM version that Intel's BIOS worked, but I'm not going to find out. Specially since all works great now.