Do you have an SSD at the moment? If not, I doubt you'd notice much difference between them, as they'd all be an immense step up from a hard drive, even the cheaper ones (Crucial MX100 for example).
The 8GB will be fine, but try not to mix and match sticks. For example 2x 4GB + 2x 8GB ought to be fine, but try to make sure the sticks are arranged correctly (the BIOS manual would tell you where to put the identical sticks to keep dual-channel mode - the slots are often colour coordinated anyway). 4x 4GB would be fine too obviously, but try to keep the identical sticks in the same channel.
The PSU should be OK, though I'm not sure how well it would accommodate a CPU overclock. The GTX 970 is a frugal beast. Same or better performance than a 780 TI, but at a fraction of the power. I went for the ASUS 970 as it turns the GPU fan off completely if the GPU temperature is under 40c (which it always is at idle) and is almost silent even when gaming. On that note, the recent ASUS motherboards are fantastic too, with awesome fan adjustment, either in the BIOS or Windows. It can automatically create profiles which will turn the fans off entirely under a certain temperature (if the fan allows it, which the software tests), and ramps them up gradually if needs be. It really has to be seen if you're after a quiet system.
I'm not suggesting a massive overclock, just bump it up to around the speed of the i7-4790K, which *should* be perfectly stable without any extra tweaks. Then you're just missing hyperthreading, which will have zero effect on 95% of what you do, and a minimal effect on video encoding (lets say a video might encode 10% faster, which in the grand scheme of things, isn't worth it, unless that's your career). Like I say, if video encoding time is that important do it properly and go Hawell Extreme.
I have to say that if I was buying a full system or on finance, I'd go for a custom built and overclocked system from Scan.co.uk or Overclockers.co.uk with the i7-5930K (6 core plus hyperthreading), but if I was simply upgrading I'd struggle to justify it personally. I want one of those ASUS motherboards, but a full system upgrade (motherboard, CPU, RAM) is a bit over the top for some fans which turn themselves off and on! I'm still rocking a i7 920 (price comparable with the i5-4690K) from 2008 and looking at benchmarks I don't see why I'd upgrade it - that's how little progress has been made with CPUs.