you don't really need an unlocked multi since the duals do 500mhz fsb
that equals this potential:
e8400 = 4.5ghz
e8500 = 4.75ghz
e8600 = 5.00ghz
If you can go any higher than that, then you probably have other techniques that won't become multiplier limited. But for all other basic overclocking purposes, 9x, 9.5x, and 10x multipliers are more than enough to get a ~ 4ghz overclock.
The e5xxx and e7xxx have even higher multies