The goal: a solid console-replacement HTPC made of parts replaced in my main gaming PC. The result: a $300 solid gaming/home theater PC. Compromises were made, but the performance is very nice for the price.
The main component left over from the main PC was the GPU. In a gaming PC, this is by far the most expensive and most important component. The Radeon R9 290X is still, as of build time, a solid 1080P 60Hz performer. I also pilfered my old 180 GB SSD (since replaced by a pair of Samsung Evo 850s in RAID 0). The storage size is obviously constraining, but the speed should be nice for this budget build.
As for the rest, I was hoping to get a decent used PC and salvage its parts. I got a Dell OptiPlex 990 locally, which turned out to have no RAM and a broken HDD. Also, the small-form-factor case did not fit the GPU, and this particular card is VERY power hungry. So, replacement parts I needed to find so far: case, PSU, RAM, and optionally, HDD.
Originally, I intended to use the motherboard on the Dell in my build, but after reading some horror stories online, I went with an Intel board. I was, at least, able to recover the DVD burner drive from the Dell. Fortunately, the i5 2400 in the Dell worked fine, and seems to be a fine gaming CPU. My gaming benchmarks are in the screenshots provided; 3DMark puts this system right at the VR-ready level. And here is a link to User Benchmark's general score:
My main gaming rig is here: