At Tom Bombadil's House...

I am on my second journey through The Lord of the Ring's; The Fellowship of the Ring. I just finished reading Chapter 7, when Frodo, Sam, Merry, and Pippin stay at Tom Bombadil and Goldberry's. My question's are, what is the significance of Frodo's dream the first night (it seems to be the reason Gandalf didn't return to The Shire to see Frodo off)? What about the others dreams? Bombadil then tells the gang stories the next day while it is raining. Bombadil even puts the ring on and doesn't disappear and even still, can see Frodo when he slipds the ring on. What is the significance of this? Why is Bombadil not affected by the ring? I recall something in the Appendice about Bombadil being of the first people to inhabit Middle-Earth and that Sauron's power doesn't sway the Eldar. There doesn't seem to be any further discussion of Bombadil or Goldberry again throughout the entire story. Only Gandalf goes to vist Bombadil at the end of Return of the King. What is then the purpose of these characters and the effect they seem to have on Frodo? I realize that Bombadil is an essential character if only becauses he saves the Hobbits twice but, why Goldberry? I'll stop with the questions now!!