This build is designed with an eye towards future upgrades. The i5-2500k is a great CPU, but it's OC-ability is greatly limited on the stock cooler. Get an aftermarket CPU cooler, and you'll be able to push those clocks way (not that the 2500k is slow at stock by any means). Possible options include opting for a much cheaper i5-2400 or i3-2120 processor, and while not OCable, are still great CPU. In fact, the i5-2400 is arguable the best performance/price value is $100+ CPUs. Then you could keep your eyes on an IB upgrade in the near future.
The PSU is a bit overkill (you could easily do with a 430W PSU), but the 650W (in addition to being cheap) gives you the ability to upgrade your GPU to a high-power monster and/or CF the GPU.
At present, the 6870 is a great value at $150, and will serve you well. If you upgrade, CF 6870 is more than a match for a GTX580.
For this build, and the AMD below it, you can easily get by with a cheaper case (like $30 cheap), but that's really more of a personal aesthetics question.AMD Build
Compared to the Intel build, there are two real differences. The A8-3870k series CPU is about a step down from the i5-2500K, but you get a step-up in the GPU department. The A8 can be overclocked, but it doesn't overclock like the 2500k. However, the difference between the two in games with the same GPU won't be that significant, so the extra GPU horsepower in this build adds a lot of value.
However, the upgrade path here is lacking. Socket FM-1 is targeted towards the medium to budget performance, so there probably won't ever be anything much better than the 3870 on FM-1. On the other hand, CPU performance is getting less and less important for games, so that might not actually be a real-world problem.
Again, the PSU is overkill as setup, but easily allows for SLI with another 560ti.