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Just wondering how much those extra cores would assist, bearing in mind the software has to be specifically written for them to take advantage. Thanks.
Not much for exactly the reason you described. The only place were multi-threading excels in games is if you're running multiple instances of said game. Like Eve Online for instance. It uses a single threaded interpreter. Each instance of the game will only ever use one core. So you're better off with a dual core with a high clock speed than a quad core with a lower clock speed. Hyperthreading might give a small performance boost, but you're hardly going to notice it in real time when playing. But if you were running 3 or 4 instances of Eve Online (multi boxing), a quad core with high clock speed would be preferable.
On my Opteron chips for example, I could run 15 instances of Eve Online on 15 actual cores, with one for the OS. But just running one instance of Eve Online I won't see any benefit whatsoever of having 15 actual cores free.
As for video editing and rendering and photoshop, unless you're in a situation were time = money (like it's your job and your wage depends on getting stuff done within a time scale), I really don't see the point of upgrading yet. And in any case, most of that software uses GPU accelerated tools. Photoshop uses GPU acceleration. So does lightroom. Blender uses GPU acceleration. Handbrake is capable of GPU offloading, but will still predominately be CPU based, but again, unless you're in a situation were time = money, there's really no point in upgrading just so something encodes 30 seconds faster. The price you'd pay forking out for new chips, board, ram just isn't worth it. It would be a different story if you were upgrading from a Phenom II, Athlon or Core 2 or something.
In the above circumstances would I be better concentrating on more ram, a couple of good SSD's and going from GTX 980 to the TI and then think about the cpu in one to two years?