i have what is now what is mostly an old setup, comprised mostly of secondhand parts. i'm still not seeing a compelling reason to get a new machine, so i'm thinking of keeping this going a while longer, but possibly with a (used) video card upgrade. i'm assuming i can go two levels up from the GTX 670 i got (used) two years ago: http://www.tomshardware.com/reviews/gpu-hierarchy,4388.html
example: just tonight i saw a GTX 780 on Craigslist for $150 (replacement card shipped from EVGA, never opened) - would something like that be worth upgrading from a GTX 670 on an i7-960 setup? i was thinking the [GTX 780, 780 Ti, 970, Titan] tier would be best, but maybe i could go [GTX 1060, 980, 690, Titan Black] or higher if i overclock.
another example: i also saw on craigslist a GTX 670 for $90, which i probably could SLI with my current card.