Is 450W PSU ok for GTX 970 ASUS STRIX

it can only deliver 400W on the 12v rail and is overall poor quality unfortunately.
without overclocking it will be alright in terms of watts, but you might wanna replace that PSU in the near future

generally for GPUs I'd go with a
https://www.amazon.co.uk/Sapphire-11260-02-20G-Nitro-Memory-Graphics/dp/B01IQS6QE6/ref=sr_1_1?s=computers&ie=UTF8&qid=1473534199&sr=1-1&keywords=rx480+4gb
or https://www.amazon.co.uk/MSI-GeForce-GTX-1060-3GT/dp/B01KHWOB78/ref=sr_1_1?s=computers&ie=UTF8&qid=1473534249&sr=1-1&keywords=1060+3gb

they will provide better performance at the same price