It is presumed that your question is aimed at minimizing Electromagnetic Interference (EMI). The answer is VERY difficult to be precise for your case. EMI depends on many factors, however, for your case the variables you should be interested in are: the separation between the cables and for how long the two cables run together. Separation is inversely-proportional (not linearly though) to EMI, and distance run together is proportional (ibid on linearity) to EMI.
It also depends on what you connect to the power outlet! If you running a washer or a vacuum cleaner, I would expect to see more re-transmissions in your Ethernet cable as a result than if you were to plug in your computer to the power outlet.
Unfortunately, the metal box won't buy you much. (However, if you do, make sure it is grounded to the same ground as the power cable.) The damage is really done in the UTP running parallel to the power cable vice where it is terminated. In many shipboard design applications (where there are a lot more confined spaces), for 120V power it typically only requires 1 inch of separation between 120V power and Ethernet cable. So, I would advise you to skip the metal box and try to provide that one inch of separation I mentioned above. You might also consider rerouting the cable if you are still concerned.