Yes NB GTL Ref sweetspot changes when other parameters are changed, and it can be a very influential setting. Also, I found that I was Linpack/Orthos/Memtest stable at a certain vNB, but in games I would sometimes crash to desktop as if I was very unstable. Crysis was the best at doing this, sometimes it would only last 30 seconds before it crashed. Even increasing vNB from 1.30v to 1.45v did nothing to help this situation, nor did changing clock skews ram skews, or NB DDR Ref's or CHA/CHB Refs. I finally dropped my NB GTL Ref to -100mv, and although it only passed 2/6 tests in linpack, it was absolutely fine in crysis, no more crashes. I then bumped it up to -80mv, and that made the system totally linpack etc stable again, and was still totally stable in crysis.

I have found on my M2F, I really only need to use cpu/NB clock skew when I am over 540 - 550 FSB. Behaviour is different with quads though, some guys need to start using the skews around 450 FSB.