http://forums.tweaktown.com/asrock/59292-asrock-z68-extreme3-gen3-compatibility-gtx-970-a.html
Seems you should be alright. Been googling, no universal issues as far as I can tell. And a (quality) 620w should be more than enough for a single GTX. (I'm using a 430w Corsair with a 670, if that means anything)
You should be fine going to a 970. I have a 2500k myself and went from a 660ti to a 970. It was a huge improvement.
If your 2500k isn't overclocked though, you should do so. Get it to 4.2ghz or higher and you'll be set for awhile.
The 620 PSU should be ok as well.
I would avoid the 960 personally. The specs just don't look that great. The 970 is a much better value and will have much longer legs.
Fantastic, a thousand thankyous, it shall be done. I have a spare nocturna cpu cooler thing laying around and didn't install it in this build (4 years ago now!!), when I crack her open for the 970 swap I'll replace the stock cpu cooler and see if I can get over 4.2ghz. Thanks again!