I would like to second David's suggestion on using the http://www.bahn.de website for train travel planning. They have a link for "International Guests" that will get you to the English version. I have planned more than one European trip using that website, and it is a great source of information.

I would like to suggest that booking ahead of time may not even be necessary, unless you are traveling on a holiday or at some particularly busy time. I found the bahn.de site to be very helpful so that I could plan my itinerary by knowing how long it would take me to get from one city to another. Plus, once I got to the train station I knew what I wanted to ask for without spending a lot of time figuring out the schedule, etc.

If you've never traveled by train before, you may want to be aware that even in the sleeper cars you are likely to be awakened by the border patrol wanting to see your passport in the middle of the night. At least this is what was happening six years ago. I took night trains from Budapest to Krakow and from Poprad (Slovakia) to Prague. Frankly, I didn't get much sleep on either train. The border guards reeked of alcohol, and they were not the friendliest folks I met in Europe. It totally freaked out my friend who was in her 60's at the time. (I was 40 at that time.) I didn't think it was nearly as upsetting as she did, but it was not a pleasant experience. To his credit, there was a nice conductor who showed us how to lock our door on one of the trains. He warned us that there could be thieves and that we should keep the door locked. I'd probably travel by night train again, but I would not expect it to be a way to save time by traveling-while-sleeping as I did back then. I just wouldn't expect to get any sleep, only to get from one place to another.

Sincerely, Barbara Roanoke, Virginia, USA