I guess my main question is how do I determine what’s overkill and what’s not? I’m speaking on a strictly gaming machine, not doing any major 3D editing, but it will be used for multimedia like movies and such.
Research. If a hardware review place is worth it's salt, it'll do something on the side rather than review hardware or post news on the latest and greatest. Hence for me, my go to websites are Anandtech and Tom's Hardware, but more on Tom's because they actually like to experiment once in a while in order to solve a gamer's greater mysteries on computer hardware and getting bang for the buck (and I'm inclined to believe their website is geared toward those who want to get the most out of their money). Otherwise, there's no real method. We can't derive raw throughput anymore and what we usually get from the hardware vendors is just best-case benchmarks used for marketing purposes. Every environment is different and there's really no strictly apples to apples comparison.
That aside, when it comes to gaming, the #1 bottleneck is the GPU. This one you can't really go "overkill" unless you have a processor that's the bottleneck. And even then, you can go as low as Sandy Bridge based Core i3 tier processors before a bottleneck is found, at least for a single video card setup. If you're not going to upgrade the GPU in say a year or so, you could get away with a Core i3. However, just to ensure consistent performance, a Core i5 would be a better choice. As for going with a GTX 680, I wouldn't consider it overkill at all. The problem is that most games made for the PC are made with consoles in mind and only a few actually use DX11 as its meant to be used (otherwise they just use it so they can offload some things to the compute shader). Hell, if you're not going to touch the latest stress testers, a $150 video card will play every game at 1080p on max settings while keeping up at least "smooth" frame rates just fine.
I'm probably rambling now, and didn't even answer the question as intended.