Use a GPU comparison site to work out what models will give you a significant improvement and then check new and second-hand prices. I suspect the only 2nd hand Nvidia candidates are the 1070 and 1080, plus Ti versions. I would make sure any replacement has increased VRAM.
Just thinking out aloud here. My machine contains:
What would be the cheapest way to boost graphics performance for this? Find a used but better graphics card on ebay?