Using a higher resolution screen is not "necessary" you will just be able to unlock smoother game play/FPS with bigger screens.
What I mean is if you only had one 4870 then a 1920x1080 screen would have great FPS but with a 2560x1600 screen it would be laggy all over.
A 4870x2 will have much better FPS on a 1920x1080 screen over a single 4870 but will have great 2560x1600 performance. Conversely if you only had a 1440x900 screen then a 4870x2 is "overkill" and I would say save your money and get a single 4870.
24 inch to 28 inch are all 1920x1200/1920x1080 displays, only that the pixels are bigger, there are LCD's like NEC's that have a weird measurement (29.8 inch, 2560x1600).
There's nothing standard between 1920x1200 and 2560x1600.
I like the Asus screens on newegg, note that most of these are way below your price range and all the 1080p screens in the second link will give you the same FPS regardless of the screen size (in inches) as sabot has mentioned, so might as well save and get a smaller screen that has the same resolution, but thats just me.