Quentin, I am running Win XP Home, AMD 3000+, 512MB Ram , NVidia 6600GT 128MB card and B4 runs miles quicker than B3 for me.
When I make changes I hardly have to wait at all, whereas with B3 it was like wading through treacle.
I am just hoping that the final product is not too expensive because I will buy this without a second thought.