Huh, exotic. The GTX 690 is basically one card with two 680 GPUs. That sounds very powerfull but it hardly depends on the games if they can take an advantage out of such a dual-gpu setup. Depending on the game even an 680 outperformes a 690 due to it's running at a higher clocks.
Also the 690 shipped with 2x2GB or 2x4GB of VRAM. This might also sound nice, but the 2x4GB is not nearly comparable to a current card with 8GB, as each 4GB is for one GPU only. One GPU can't access the VRAM of the other GPU, even if the second GPU is not used at all. That means all the data needs to be stored for every gpu in each VRAM. So this is really 2x4GB and not 1x8GB, as you need to store everything twice. So effectively you have only 4GB.
Both that points are basically the reason why NVidia discontinued SLI and especially those dual-GPU cards.
I have no clue how you come to 6GB, maybe there have been some custom designs that shipped with 2x3GB i never heard from.
According to all of this, an upgrade to an (used) 1060 (6GB) or RX580 (8GB) would also increase your perfomance by estimated 50% while halfing the power consumption (690 = 300W, 1060 = 120W, RX580 = 185W).