It's usually easier to fully attach it outside the case. But if your mobo has some screws that are unreachable when the HSF is on, it might work better to screw the mobo into the case before attaching the HSF, probably not the case though.
I would put it "east/west" airflow because you want that little bit of airflow going over the fins on the heatsink for your P5B and directly out the back of the case. Any air that doesn't go directly out the back will be warm and rise to the top fan and out. If you had it "north/south" it might eddy around the top fan and circle over the memory. No big deal though, either way should work just fine.