In reply to Goofyfoot:
Have you used the buffalo before?
Have you done much spring and autumn hill walking?
None of the ventilation systems including undoing the front zip (with a back pack on) really work for me, normally I remain too hot and sweaty, if not I'll likely be too hot in some areas of my body and too cold in others!
For spring and autum generally a hard shell top and bottom in case it rains, coupled with your standard walking clothes and maybe an extra fleece will suffice. As you get used to that kit you can then expand your kit based on how your body performs and the kit you have is working.