I guess the 'au' is Australia?
If so I know stuff there is stupidly expensive but $280 for a old card like the HD7970 is too much, especially if you can get a brand new R9 280X for the same amount.
Overall the R9 280X is slightly faster but the differences are small, if you can knock the Craiglist seller down to a more reasonable price the HD7970 is still a strong card, with either you'll be looking at high/max settings in most games at 1080 rez.
At 1440 they'll both struggle to maintain smooth gameplay without dropping the settings to med/high, but it will vary a lot according to the game and exactly which settings you tweak: As a long time AMD user I can say the best way to hammer the frame rate is to apply more than 2xAA for example.
No card for <> $300 will run games at 4k res, even flagship cards like the R9 290X and TitanX need to be run in a dual card SLI/CF configuration to handle 4k res well.
The above comments apply to the GTX960 as its overall performance only a little below that of the R9 280X.
Before jumping in, bear in mind the AMD cards will typically need 1 6 pin and 1 8 pin PCI-E power leads to run and a good quality 600Watt power supply while almost all the GTX960 cards need a single 6 pin PCI-E lead and 500Watts.