I'm a grad student, and recently my laptop (my only computer) crapped out on me in the middle of a term. This was terrifying, as it took 9 days to repair through the shop. This relegated me to using the public library's PCs to do my work, so my main motivation for building a desktop was to have a second workstation to use in the future. From there, I decided on a budget of ~$600 and figured I might as well make the fasted machine I could at that price point. These were my parts rationalia:
-CPU: I struggled for a while with this choice, as I was constrained by a budget but also wanted enough power to feed my GPU. Most of the games I will play use only 2 cores, and the rest run well on virtual cores (i.e. the i3's hyperthreading technology) if they need to. In the end I decided against the i5 due to it's 50% price bump from an i3 w/o a commiserate 50% bump in benchmarks I care about, but also against OC'ing a G3258 due to its lack of hyperthreading and therefore a lack of some measure of futureproofing. Plus, I have a clear upgrade path since I'm running an L1150 socket.
-GPU: I got this refurbished, and in the end I think this was an absolutely absurd deal. I got a $250-$300 card for $165, which includes a 2 year warranty. It's more than enough to run my games at 1080p and 60fps at maxed out settings (e.g. Metro Last Light, with everything except supersampling maxed and it rarely dips to 55fps).
-RAM: Cheapest 8gb set. Don't need 16gb for gaming. RAM is RAM. It works, 10/10.
-PSU: Found a great deal on NewEgg for a modular PSU from Corsair, which is a respected company in this field. The 500W is high for my current build but gives me overhead for future upgrades.
-Mobo: I was looking for a feature packed budget mITX motherboard, and ASRock's had the best reviews. Socket L1150, on board audio and WIFI (clutch, as an mITX's PCI slot is reserved for the GPU). Overall I'm happy, but the wiring is a little haphazard.
-Case: I really wanted to make a small form factor PC, and CM Elite 130 really caught my eye. It's really cheap, rather small, and provides a deceptive amount of room to maneuver inside. Great cooling arrangements, though the fans are a tad loud so though might get replaced soon.
-Storage: I salvaged an M4 SSD (purchased in 2012) from an old laptop, for which it was bought as an upgrade. Obviously not competitive with today's SSDs, but still plenty fast.