https://pcpartpicker.com/product/cCDzK8/evga-geforce-gtx-1050-2gb-sc-gaming-video-card-02g-p4-6152-kr
Its $100, and it will help out a lot in games, but your cpu limits you from getting a really good card.
Your mobo is fine, though a newer Z series board would overclock better, but it wouldnt really be worth it. a SSD would be the single best upgrade, but other than that, I would just save up for a new build.
yeah, it may be because the ram itself is fine, but the cpu doesnt like it when they are both stressed. Ive had a very similar issue i solved by turning the ram down from 2133 to 1866.