In an interesting turn of events, it works now after installing both GTX570, which switches the PCI-E slots to x8 mode.Considering the SLI setup was the intended outcome, an unsatisfying solution, but one that works. At last.
Bad behaviour to start one's first forum post with slight thread necromany, but I'd really be interested if someone helped the thread opener (via PM or so) since I am experiencing the very similar (same?) symptoms and would like to check for the poss...