Before my blood thinned during 6 years in Costa Rica's marvelous climate, in Ottawa, Canada I NEVER went to the extent that Leon recommends. Never found it necesary to put anything over my pants. Used a nice sheepskin jacket in the coldest part of Ottawa's winter, and mittens or good gloves. No hat. Galoshes over my normal shoes. That was enough.
But while in Vancouver (I think Orange Country is headed for Vancouver, no?) last December, I still needed only a wind-proof jacket and sweater underneath, no gloves, and certainly no hat (I hate hat hair!!). I wore tough shoes, but no boots.
ANyway, I'd wait until arriving, take a week or two to decide what you really need, and look for sales rather than load up on clothing you may never need while still in the USA. You're likely to find a better range of clothing here than over the Internet.
A key component (if you're going casual) is an outer layer (whether wind breaker or fully-insulated jacket) that cuts the wind and still breathes. Like Gortex or equivalent).
I always test the fabric by trying to blow through it from the outside (should block your breath) and from the inside (should let your breath pass through).