If the 2500K is healthy, it's a no-brainer at that price delta!
This is sort of like the JFK assassination: There will never be a consensus about it. It is unlike that "mystery" for the fact that over and above any consensus or that most things "happen only one way," the OP has a set of personal preferences for what he wants and what he wants to spend, given all the expected costs and benefits.
Frankly, watching the frenzy here over Cinebench scores, I can see that I'm "future-proof" myself with an SB-K chip for the time being. Maybe that works for the OP, but only he can answer it.
Here's something else:
http://www.staples.com/Lenovo-H530-D...5#desc_content
Used to be the case that I could actually save money building my own system. There's still the benefit of a learning curve: if you build it, you can likely service it. In thirty years, I've never taken a computer in for "repair under warranty," never had an extended service agreement, never had "Geek Squad" fix anything or install anything for me, and never needed to avail myself of "data resurrection" or "virus cleaning" services for $130 per issue.
But it begs the question: Whaddaya want? What can you get? How do you want to get it?
As for the Staples non-K bargain, you can't overclock it, can't upgrade the PSU, can't do a lot of things or so I suppose. But your average Mainstreamer on the street probably feels good to get a computer for that price.
And I have to wonder why I'm willing to spend three times that for an overclocked, SSD-bootable speed-demon.
I suppose I could even buy the "previously-owned" i5-2500K myself and stick it in my parts locker in the event that something goes wrong with my 2600K. But let's face it: that ain' gonna happen. I'd do better to pick up an inexpensive Z77 motherboard to put in the closet . . . until that day . . .
But the OP wants a "current" computer and upgrade from what he has -- not necessarily a new one.