there is no overkill on pc gaming, 780ti is the best you can get, sli does not count as its do not work on everything.
titans in any form are gaming productivity hybrids and do not give much extra to games.
i5 4670 and 780ti is about the best you can get right now until devils canyon comes.
so many games drop below 60fps with said hardware and therefore can not be overkill. 144fps gaming or higher resolution gaming does need even more power.