I have many problems on Civ V, huge maps/marathon speed, post industrial era, with an GTX 770 2gb even on FullHD (1080p). It actually crashes in modern era. On 1440p is even worse. And the main reason is lack of vram, so try to buy at least a 4gb card. GTX 1060 or RX480 8gb (even better as game will be optimized for AMD cards). For an under 300 budget RX 480 is the top choice if you will play a lot of Civ6 and latest Total War games. At about 400 there is 1070 which is a great card for anything. If you can fork money for one, you will be set for quite some good 3-4 years, even on 1440p.