It will vary from game to game.
The 2650 v1 is, I believe, a Sandy Bridge 8-core / 16 thread CPU. It's based off of Intel's 2nd generation Core architecture, and has a base clock of 2.0ghz and a boost clock of 2.8ghz.
Most games don't make very good use of more than 4 threads, so something like 50-75% of your CPU will be idle while gaming. Games really like fast cores, and Skylake cores are clocked almost twice as fast, with a per-clock advantage of ~25-30%, so in extreme cases, you may see an i7 6700K performing 2.5x better than your Xeon. In other cases, where all of your cores can be used, it might perform very close to a modern i7.