Originally Posted by
scrumpleberry ^ Gramatically anal-retentive people, we're calling you to our aid..!
I'm here!
Em dashes are—for the most part—interchangeable with a colon. They can also replace parentheses or two commas used for apposition.
The only incorrect sentence of yours is the one including the comma. DO NOT JOIN TWO INDEPENDENT CLAUSES WITH A COMMA!
What's an independent clause? Well, "You should read this," and, "It's my favorite book," are both independent clauses because when seperated, they are still sentences.
Also, with your question on capitalization after colons, the American standard is to capitalize. The British standard is not to capitalize; I believe.