the 4.8-5.1GHz overclocks can be done with about any sandy bridge chip, but if you look closer the majority of them use realy high voltages. mine is running stable at 4.5 with 1.26 vcore, with a vid of 1.18. i could get it stable at 4.8+ but the increase in needed vcore is not realy worth it to me. yes, 1.4 or even 1.5 is still in intels "vid range", but you allways have to look at the offset towards the default vid of your specific cpu. 1.4v on a cpu with 1.1v vid might be stable, but will decrease the lifetime of that cpu a lot more than 1.4v on a cpu with 1.3v vid. from what i have seen 4.2-4.5 GHz is a much more realistic value for "safe" 24/7 settings.
anyway, the performance is still pretty awesome at "just" 4.5GHz and with the current prices you get a lot more performance for your money than with any first-gen i5/i7 system