well if you wanna run BF3 ON MAXIMUM i suggest a 1440x900 or less a bit what are your other system specs?
1920 X 1080 res monitor is the best buy. The r cheap now a days. 7870 can play most of the game at 1080p resolution at high setting. price difference between 1080p and lower res monitors are not that much. so, it is better to buy 1080p res monitor.