Thinking of choosing a gtx 760 over a 770 due to the £60 or so I'll save, how much of a performance difference can I expect on games such as borderlands 2/battlefield 4?
You can find benchmarks on anandtech.com. If I recall correctly, the difference in FPS is around 20 to 30%.
If you're using a 1080p 60Hz monitor, you will see no difference at all in Borderlands 2. However, on BF4, the 770 will be constantly at 60 FPS on ultra, whereas you'll have to back it down to high with a 760.
I personally can't tell the difference between High and Ultra in BF4.