See Tweakers Page - Exporting Style , Tweakers Page - What video card to use? and Tweakers Page - Balanced Systems
A Quadro K5000 at € 1800 performs almost equal to the GTX 760 at € 200. Around 11% slower than the much more affordable 760.
The GTX 760 is almost as good as the current GTX 680. For exporting the video card is largely unused. It is a matter of the CPU. Read the first link.
A GTX 980 can be useful if you have a single overclocked 6+ core CPU or dual 6+ core Xeons with lots of memory, 64 GB and very fast raid arrays. In all other cases the 980 is overkill.
With only 24 GB memory, I assume you are talking about the first gen i7-9xx CPU, not the i7-39xx or i7-49xx. If this assumption is correct, then keep your current video cards. It is a waste of money to get new video cards for what can at most be a very marginal performance improvement on rendering for preview and no benefit at all with rendering / encoding for export.