We will be in Dresden for 4 days in November...can we get a train there and travel to Prague or will it be necessary to travel back to Berlin and get a train from there to Prague
It's a 2-hour trip from Dresden Hbf to Prague by direct train. You'll pay around €35 each at the counter. If you want to pre-purchase tickets you might pay about half that - but you'll need to pin down your travel times in advance for those saver fares. Use the DB (German Railways) itinerary page to find schedules and check prices.
Yes, if you go back to Berlin, you'll get on a train that will stop in Dresden on the way to Prague.
No need to return to Berlin. I have tickets for Dresden to Prague direct in about two weeks .
Dresden is a stop on the same line that one would normally take between Berlin and Prague. It is not a detour nor a separate line.
Try and sit on the left, you'll go right down the Elbe valley.
You've gotten your answer for this particular route, but for all such questions, the easiest way to get the answer is to use the Bahn (German Rail) website http://reiseauskunft.bahn.de/bin/query.exe/en, following Rick's tutorial: http://www.ricksteves.com/travel-tips/transportation/trains/online-schedules. In particular, under number 5, see how to get a list of all intermediate stations and change points.