One note on "oversold" trains - actually (with a few exceptions), there is no such thing as "oversold" trains, as you may purchase (full fare) train tickets in Europe without seat reservations which are valid on any train of the given service category (like regional, IC or ICE) during several days (usually 2) for single trips or one month for return journeys. Hence, as many people appreciate the flexibility of those tickets, it is quite likely that more people will actually board a train than the number of abailable seats. This is particularly true when travelling on Fridays or Sundays. Of course, people will have to stand during the ride under such circumstances, both in first and second class.
Double- or multi-selling of seat reservations, however, has become a rare exception in the last few years. Hence, no matter whether you will buy first or second class, it is very advisable to book a seat reservation at a small charge (free for first class within Germany) when you buy the tickets, especially when you are travelling as a non-solo traveller and when you purchase tickets that are only valid on a specific train. Counting on a seat in the restaurant car is a bad idea, as others will have the same idea and, to be honest, it's bad style towards people who actually want to dine on the train.
Whenever you find your reserved seats occupied, try to politely ask the people sitting there to move away (after double and triple checking your reserved seat number, coach number, train number and date of travel). If they refuse, get a conductor to sort things out.