We dont known how zen or Ryzen will perform yet. There are no benchmarks, besides a few render tests and that was likely a very expensive cpu since they were comparing it to a thousand dollar i7 5960x. The mainstream ryzen processors will likely be 2, 4, and 6 core with hyper threading, those have not been talked about yet to my knowledge. So I dont know if its worth a buy we will see in the coming weeks, but the 8 core cpu you are talking about will more than likely cost 700 dollars plus.
A lot of games utilize more than four cores battlefield 1 and GTA V for example. However single thread performance is very important in most games, AMD claims they have made large improvements there.