I suggest getting a Radeon 7850. Radeon 6800 cards are prone to significant stutter issues when used in Crossfire, a common issue with graphics cards that use a VLIW5 architecture such as all Radeon 5000 cards and all Radeon 6000 cards except for the Radeon 6930, 6950, 6970, and 6990.
You have a low resolution, but don't let hytecgowthaman convince you that this can't be helped with a decent graphics card such as the Radeon 7850. Simply pump up the MSAA settings and max out texture settings and you're good to go.
Also, monitor size has absolutely nothing to do with performance. Monitor resolution is important, but that is not directly correlated to size.
I don't suggest going with a Nvidia card at this price point unless you find at least a GTX 660 that isn't much more expensive than any Radeon 7850 that you can afford.