Yes, it looks like my machine is having an error.
... or bad RAM.
Thanks for chiming in and for the suggestions.
Before reading you reply, I tried swapping all three memory sticks (3x 1 GiB DDR 400 @ 333 MHz) by other three actually older sticks, but known to be good. Done it because it's the easiest thing to do. If the BSOD persists, I'll move the HDDs and PCI cards to another identical motherboard / processor assembly (which is where the removed RAM sticks are going to be after cleaning). If that also proves useless, then I'll get the full dump.