There is VERY little on any of the boards regarding South America. We are booked on Mercury, Rio to Valparaiso departing Rio on 1/6/02. Just remember that the temperature range will be great. You can expect 80's in Valparaiso and again in Buenos Aires, but it is likely to be in the 50's when you round Cape Horn. Also, because it is summer, the days will be very long, nearly 18 hours of sunlight. The seas can also be rough around the cape. There is a good chance that you will encounter some rain.
We have gotten some good information doing a good search using one of the web search engines-- yahoo, google, etc. Search under the names of each of the ports/areas you'll be visiting. Remember also to search under "Patagonia" "Ushuaia" "Tierra del Fuego" and "Peninsula Valdes" I would be happy to share the sites I have found with you.
You will also find that there are far fewer guide books available for South America. I have found the "Insight Guide" for each country and major city to be very informative. Your local bookstore or travel bookstore might have them, and I know Border's carries them. You can probably also find them on Amazon.com.
Part of what attracted us to this itinerary is the incredible variety of what we'll be experiencing. This is definitely not what you would get on a Carribbean ininerary.
We are looking forward to a real adventure.