In order from quickest to obsolete to slowest imo:
Case
PSU
Regular Hard Drive
Solid State Drive (prices drop steadily, but performance isn't really going to get better without a new tech breakthrough such as sata 4)
Mobo + CPU + RAM
GPU
Unfortunately if you are trying to buy a functional PC, parts at a time, that mostly wont happen. as you need at bare minimum, 6 of these parts to function. (case, psu, cpu/mobo/ram, hard drive]
But if you want to get some costs out of the way while waiting on more money, then i'd buy up case, psu and storage first, as these will not become outdated anytime soon (soon relative to the time it takes to amass 1000 dollars)
As of current, AMD releases new cpu's this year, and intel will release another round shortly after (in 2017). while these will not necessarily bring any groundbreaking performance, they should hopefully bring good stuff down cheaper.
New gpus from both AMD and nvidia coming this year, so definitely buy those last, as I would expect them in about 4 months.
So definitely save all of that for last, in that order. (buying newer gpu is more relevant than buying newer cpu, insofar as gaming is concerned.
This post was edited by Rapture on Apr 7 2016 10:48am