You shouldn't compare 24" @ 1920x1200 with 2 21" @ 2048x768. Both the 21" should at least be capable of running 1280x1024, so that would make 2560x1024 to compare with. Find a 24" that has that resolution and then compare again. Also remember that 24" is measured across the screen. 2x21" is a lot more screen surface.
Just wanted to give you all an update... I went to the local mall and brought up my charts on the dell 24 inch monitor and it looks like I will lose about 10% horizontally but gain about 25% vertically.... So it appears as though what I can get on the 24" monitor is what I currently have on 2 20" monitors... given the difference between 2048by768 vs 1920by 1200 on the new 24" monitor. thanks all again for your inpug toucan
unless you have a seeing problem, i don't understand why you are using 2048x768 on 2- 20" monitors. if you have to use that blown up res, you can save yourself a lot of money by using 17" monitors.