The statement about whether something is overkill or not needs to be taken into proper context. I am well aware of the fact that technology advances quickly, thus "future proofing" is a prime consideration that many builders must make. However, you need to consider each part individually here. Since I am the one that stated that the i7 was overkill, allow me to explain my reasoning here, though it might get a bit technical and off topic. Nevertheless, this is an example of knowledge that I have learned over the years, and why it is a good idea to thoroughly research this stuff on your own.
Throughout the history of x86 CPUs, there have been periods where the bottleneck for most systems would alternate between hardware and software. You might remember your old Pentium system from the early 2000's to be rather slow and unresponsive, even back then. This was a point where CPUs have more or less reached a barrier in internal clock speeds due to thermal limitations (And CPU Instructions Per Clock Cycle has only increased marginally with each generation). That all changed when AMD and later Intel released dual core processors in the mid 2000's (Before this moment, AMD's CPUs were in fact superior). Software was beginning to become "multi-threaded", that is, optimized to take advantage of the multiple cores found in newer processors to execute certain tasks simultaneously. A majority of software in existence today has 1-4 threads on average, with the exception of certain professional applications like CAD and Video Encoding software, which can utilize 8 threads or more.
What does this mean in regards to an i7? The truth is, a quad core i7 and i5 are very similar internally. The only real difference between them, and why the i7 is at least $100 more is hyper threading. This technology allows each core in the CPU to execute two threads in parallel. Hence, an i7 appears to have 8 logical cores to your operating system. This might sound like a great thing, but as I said, most software does not have enough threads to take advantage of this technology. Unless you are in the group of professionals I mentioned in the above paragraph, you will never see the benefits of an i7 over an i5 with the same number of cores. You might think that software will become more multi-threaded over time, but this is unlikely to happen. The i7 920, which was released 4 years ago, was one of the first quad core CPUs to have this technology, but amount of software utilizing it has hardly increased since then, and currently it looks to be that way.
What does this mean in regards to an i7? The truth is, a quad core i7 and i5 are very similar internally. The only real difference between them, and why the i7 is at least $100 more is hyper threading. This technology allows each core in the CPU to execute two threads in parallel. Hence, an i7 appears to have 8 logical cores to your operating system. This might sound like a great thing, but as I said, most software does not have enough threads to take advantage of this technology. Unless you are in the group of professionals I mentioned in the above paragraph, you will never see the benefits of an i7 over an i5 with the same number of cores. You might think that software will become more multi-threaded over time, but this is unlikely to happen. The i7 920, which was released 4 years ago, was one of the first quad core CPUs to have this technology, but amount of software utilizing it has hardly increased since then, and currently it looks to be that way.
The point is, most software, including most games, aren't that taxing on the CPU these days, and it has been that way for sometime (arguably since the beginning of dual core processors, in fact). I don't expect to see any long term benefits in regards to buying the most powerful CPU now other than the advent of 4KHD video, though it is still at least five years away from being mainstream. I have an i5 2500K in my system, and I expect it to meet my needs for at least 4 years.
Keep in mind that this is just considering one component. The argument would be more difficult to make for video cards. Unlike the CPU, games are very demanding on your video card, so there is some benefit to having the most powerful video card available now. Once again though, I stress that context is important. Graphically, PC games have actually not improved very much in the past 4 years, due to the prolonged lifespan of consoles. For a while, Crysis, despite its age, strained the video card more than any other game until the very recent release of Battlefield 3. Still, if gaming is a primary consideration, this is one area where I would pay close attention to in regards to future proofing.
