I think if we take emotions out of the equation, for gaming purposes, a CPU platform is worth upgrading when your CPU either holding back your GPU or you managed to sell your old platform for a good price so that an upgrade is very cost effective. Otherwise, the primary reasons to upgrade the CPU are to take advantage of faster PCIe SSDs or get more CPU cores to reduce render/encoding times, etc. In your case, the FX6300 is already outdated if we consider that it holds back cards like 390/970 in some games, and especially cards like 980Ti. You can always crank MSAA though and increase the load on the GPU but in CPU limited scenarios it's going to be bottlenecked either way. That's why a 390/970/290 is probably the highest level GPU I'd even recommend, the cheapest of those 3.
Don't forget R9 290/290X. Because they have a tarnished reputation their prices
keep dropping. For your CPU, whichever you can find the cheapest among R9 290/290X/970/390 is the card you should get. By the time you upgrade to Intel+DDR4 in 2-3 years as you said, all of these cards will have been surpassed by something in the $250 range. I mean think about it November 2013 R9 290X cost $550 and GTX780Ti cost $700 and today that level of performance can be purchased for $280 without any sale. Therefore, there is no need to overspend $60-80 extra for 5-10% more GPU performance your CPU won't even provide to the card.