DHTML vs. Flash
I'd be nuts if I didn't make a shameless plug right now for my Flash book, Advanced Flash 5 for Web Professionals. It's got everything you need to know about ActionScript. I had a blast writing it, and you might like it too.
The decision to use Flash or DHTML rests primarily on your target audience. Are you creating a bitchin' cool site for teen girls or a brochureware site for a marketing consultant intended for corporate CEOs? Higher-up executive types tend to not want to spend valuable time downloading plug-ins, while teens are more likely to. It all comes back to your audience and their typical behavior.