You don't really need to know that much to get started, but you can't really completely ignore core JSF concepts either. I recommend not getting too bogged down by JSF details at the beginning. Most JSF books/articles jump way too quickly into irrelevant details, leaving people with a bad taste in their mouth.
Before I started seam. I've already learn JSF, but no facelets. Learning JSF could be worthwhile. But here's some tips for you, if you really want to learn seam you may skip the navigation part of JSF as Seam has a nice way in dealing with this. You may also choose not to get deep into managed beans in JSF, because using Seam will do those things for you. With Seam, JSF just got easier and fun to develop. Although I just have only a week of experience.
As for facelets, I just learn it by examples from Seam and the code generated from "seam gen" and refers to some facelets article and the experience so far is nice!
Facelets... nice.... now that is an understatement. Did you ever try jsf in jsp and do some advanced stuff.... it's a hell. Facelets makes this a breeze.