Your 20 degree bag will be fine. Make every effort to keep your gear dry! Rain can fall at any time (and for weeks at a time!) and the air is so damp that nothing will dry out once it gets wet. I know this from experience. <img src="/forums/images/graemlins/frown.gif" alt="" /> I use a dry bag for my extra clothes. Your footwear should be Goretex lined, as the underbrush will always be wet, even if there is no rain. The Olympics are beautiful, not a cold as the Cascades, but the dampness poses an increased hypothermia risk.

Aside from these considerations, plan to have a wonderful trip!