I'm no pro at overclocking, but i assume ur ram is preventing a more higher overclock.
From what i've read u need a ddr800 to overclock to 400fsb.
Take you cpu 1066/1550 divide by four. ur ram fsb divide by two 266/387.
Therefour your ddr667 ram at standard fsb 333 is running overclocked 54mhz already.
If you are using normal value ram... thats a lot already. Might kill ur ram in the long run.
(Btw i heard going above 3ghz is does not show much performance increase, unless u can boost it to near 3.6ghz)