Its never too fast its just that I don't want to get into the voltage thing as thats when the extra wear / danger of overheating really starts to kick in. Non voltage overclocking is generally extremely safe as your not pushing the components outside of their normal power parameters.
As for the original post, its aim really is to give people an aiming point for processor spedd if upgrading to Vista Ultimate 64 based on my experience. In other words, my experience has been that if you want decent Vista performance you want to be aiming at a processor at probably E6800 level. Its possible this may vary between setups but for me it certainly seem to be the true sweet spot.
I know increasing speed means it works better! Point is Vista is a real system hog and people should be aware of the kind of processor they're going to need. I Bought the E6600 thinking it was enough, it wasn't, it was mediocre until I upped the performance. Vista is very different to XP in its performance requirements.