My 6 core Phenom II can compile the entire MAME source tree in about two and a half minutes; it would be interesting to see how much faster Sandy Bridge's 4 real + 4 fake cores are than a true 6 core processor.
I bought the Phenom II about 5 months ago specifically to speed up my MAME compiles (there is very little that I do that cares about processing power aside from compiles), and so far could not be happier with it. But more speed is always welcome.