It's a good time I'd say. I bought my GTX670 just last month, it was an upgrade from a GTX285 (hadn't upgraded the GPU since then at all). I already had the i7 2600k and the 8GB of DDR so I couldn't find any reasons to upgrade those two, so I only upgraded the GPU and called it a night. I might upgrade my GPU/Memory/Motherboard trio sometime next year I'd guess, but I know myself enough... I'm saying this now and it could end up being an upgrade sometime in 2016 instead, heh.
Additionally, both the PS4's and the next XBOX's hardware capabilities will dictate how most future "PC games" will end up like. What I mean, obviously, is that just like the current generation of consoles the future of PC gaming's "library variety" will mostly depend on console ports (not saying there won't be PC exclusives but you guys know the drill by now and it's been the case since 2006). Whatever ends up on PC from the PS4 and/or the next XBOX will be ported, and won't display more than what the consoles they were developed on could technically do. So I won't exactly (for example) upgrade to 16GB of DDR even in five years from now if the PS4 and the next XBOX are limited at 8GB. Since most ported games surely won't use that much or even close to that maximum amount (and to my knowledge no PC games to this day, even "PC exclusives" use 4GB of RAM, and the only one in my own library that I know of using around 2GB or RAM at most is the original Supreme Commander... and I think that's about it).
Anyway, for a "mid range" gaming PC prices are cheap and components are already very good. What I would do if I had to get a "brand new" gaming PC from old components (I.E. "full upgrade" or perhaps rather just a brand new PC rather than an upgrade per se) would be to simply go for a rig that's almost identical to the hardware of the upcoming new consoles generation. We've just seen the basic first hardware details of the PS4. You might want to wait to see the specs for the next XBOX too just in case. But you can be "safe" by sticking with a 4-core (multi-threaded) CPU, 8GB of memory, a decent motherboard (doesn't need to be SLi or Crossfire capable) and a "mid-range" GPU in the veins of a GTX660 or perhaps just go with a high-end GPU and call it a night, GTX670/680 or Radeon equivalents. If you "stay in line" with the next console's hardware muscles you should be safe for the coming years, if not for the entire life span of said new consoles (since technically they won't change for perhaps another six or seven years from now, if not more).