At least with a 99.9% certainty.
It seemed that the memory upgrade I bought when I bought the MacBook is having trouble. Weird, because under Leopard it worked just fine. Snow Leopard crashes (system freeze), and sometimes even with weird flashing lines on the screen.
I guess that Snow Leopard is a bit more sensitive about the memory that's installed.
Ordered a set of Kingston KTA-MB1066K2/4G @ Informatique. Let's see if that's more stable.
The old RAM I had was OCZ So-Dimm DDR3 PC3-8500, 1066MHz (P/N: OCZ3M10664GK). Let's see if I can put it in another laptop. A less picky one.