A new GPU is a given. PCI-E 2.0 x16 won't hold it back any noticeable amount.
For sure. But at the same time I can't help but feel like paying full price for a year-old (or more, in AMD's case) GPU is a ripoff, when 16nm is "just around the corner".
For the SSD, unless you are doing something that would be considered to be a "heavy workload", then it won't be much faster, in real-world usage (as opposed to benchmarks) than a decent quality fast SATA6G SSD. Consider getting a 1TB Samsung 850 EVO.
I find myself doing a lot more copying and pasting of medium to large (5-50GB) quantities of data a lot more than I did 5 years ago as my needs have changed, and having an SSD with slower sequential transfer speeds than a modern HDD makes that kinda painful. Still, given that I can always go do something else for 15 minutes, this is nowhere near as much of a priority as a new GPU.
Is the 2600K overclocked at all? Consider getting a CoolerMaster Hyper 212 EVO heatpipe heatsink cooler for it, and overclocking it to 4.0-4.4Ghz. That should put some spring back in its step.
It's overclocked to 4.6 Ghz. When it was new it could go up to 4.8, but the voltage required was insane and after about 6 months I started getting BSODs, so I pared back the OC a bit. Also, I have the older Hyper 212 Plus already, and from what I've read the EVO is a rather small upgrade from that.
If you're referring to gaming, drop a newer graphics card (GTX960/R9-380) in there and you should be good to go until Kaby Lake arrives.
If I were to pick up a new GPU today, I'd probably go for the GTX980. Still, it's no cheaper than it was at launch a year ago, so I'm hesitant to say the least.
You might also want to consider a newer SSD. The X25M is only SATA2.
Indeed. The leading candidate is currently the Kingston HyperX Predator.
Or if you're looking for an "excuse" to scratch the upgrade itch, go for it. The Skylake platform brings some worthwhile improvements compared to Sandy/Ivy Bridge.
The "upgrade itch" actually comes in the form of ditching this desktop and switching over to a laptop - between the stagnation of desktop CPU performance and Thunderbolt 3 officially supporting external GPUs, laptop performance has pretty much caught up to desktops, and a laptop with an i7-6xxxHQ is a lot more couch surfing friendly than any desktop is.
On the other hand, I could bring this desktop back up to near top-of-the-line performance for far less $$$, and at the current (single digit percentage) rate of performance improvements with each generation, that could be enough to last me until the end of this decade...