If this is for gaming you will need to atleast go Crossfire with two of those cards if not replace it with two better cards. Running 5760x1080 puts alot of demand on the GPU and a single card will not cut it. If you go that route make sure your monitor choose has a DisplayPort input as it will save you the hassle of finding a active DisplayPort Adapter. With AMD/ATI one of the Displays must be connected to a DisplayPort or Active Adapter.
But with a $400 budget I am not sure you are going to manage at least 2 monitors and a second GPU. Also does you PSU have enough to power a second GPU in Crossfire?