It depends what you budget is and assuming your PSU isn't that great, I'd go with:
For the cheapest card I'd recommend the 7750
http://pcpartpicker.com/parts/video-card/#sort=a8&c=80
For a more powerful one that you can even reuse in a future build the 750ti would be my choice
http://pcpartpicker.com/parts/video-card/#sort=a8&c=80
Both cards will have models that don't use extra power connectors. (they don't need to be plugged into your PSU)
I know the EVGA 750ti OC is one of them, not sure on the other 750ti cards.
Because Total War is so old, either card would be a massive step up and I believe each of them could play that game on high settings.
The 750ti would be able to max out the game.