They're great fun but the non-extreme CPU's have locked QPI multi's, won't go below X36, which means when you up the BCLK you have to put up with a high QPI which can destroy an OC. With the extreme i7's you can drop the QPI speed to a reasonable level with a high BCLK.
And trying to find the maximum on the internet is hard. Some have said 7500-8000 is pretty much the limit. Which would mean a max BCLK speed of 223Mhz. There's the 'Slow Mode' (X24 I think?) setting which i've yet to try...