Also if you are on a tight budget the EVGA 500W 80 Plus Power Supply will be fine.
YOUR BASKET
1 x EVGA 500W 80 Plus Power Supply (100-W1-0500-KR) £36.95
Total : £46.55 (includes shipping : £8.00 Ex.VAT).
This will be perfectly fine, unless you really want gold rating (shouldn't be a major concern for you) and Modular.
Also on the GPU subject here's an approximate performance chart to help you understand the difference between the GPU you have and more recent GPUs.
Prices 2nd hand apart from the GTX970.
8800 GTX (£10-15) - HD5870 (£30-£50) - GTX 670 (£60-£100) - GTX 970 (£250+ (new from OCUK)
Each step is approximately doubling the performance and ram (in most cases).
That PSU would run any 1 of those cards in your system.
YOUR BASKET
1 x EVGA 500W 80 Plus Power Supply (100-W1-0500-KR) £36.95
Total : £46.55 (includes shipping : £8.00 Ex.VAT).
This will be perfectly fine, unless you really want gold rating (shouldn't be a major concern for you) and Modular.
Also on the GPU subject here's an approximate performance chart to help you understand the difference between the GPU you have and more recent GPUs.
Prices 2nd hand apart from the GTX970.
8800 GTX (£10-15) - HD5870 (£30-£50) - GTX 670 (£60-£100) - GTX 970 (£250+ (new from OCUK)
Each step is approximately doubling the performance and ram (in most cases).
That PSU would run any 1 of those cards in your system.