I suggest you look at the Building Science Corporation web site.
It discusses building approaches for various climates.
I think you will find that they do not like vented attics in your
climate.
Home Energy site is useful.

Echoing other comments:
Duct leaks and duct insulation will help a lot.
Running ducts through conditioned space looses least energy, but
retrofit will be expensive.
Shading of roof, wall, and particularly windows is very useful.
TB