16GB is not always overkill
but
16GB for a gaming rig is definitely overkill.
There are few, VERY FEW games that use more than 2GB. You're not running out of 8GB (or even 4GB) while gaming unless you're running a lot of games simultaneously and alt-tabbing between them. Or you're doing something else memory intensive in the background.
Eventually everyone will have 16GB, yadda yadda arguments:
Game tech moves slow in this regard. Memory doesn't scale with game options well, and they have to program games for a common denominator. That common denominator right now is 4GB. Any game that would benefit from more than 8GB will lock out an extremely large portion of potential customers (those with 4GB or less) due to the memory requirement, they can't really make use of 8GB but scale back so performance is 'acceptable' with 4, so games still perform perfect with 4GB.
Eventually that common denominator will be 8GB, but it's not that yet. 8GB will give you years on a gaming machine. 16GB is just ludicrous unless you're doing something else with the machine that requires the additional memory
4GB is enough for a gaming rig.
8GB is building for the future and taking advantage of inexpensive RAM.
16GB for a gaming rig is like buying a dozen roses for your computer. You're wasting money, but it might make you feel better.