Well, even if you NOW are maxing out all the quality settings in games, in the future you might have to start setting them lower. If you now go to a higher resolution, you will need to lower the settings now, and even further in the future.
Also, if you want a higher resolution, you HAVE to stretch for 27", as the 1920x1200 is currently the highest 24" has to offer (edit: this is the situation now, but it seems Dell is introducing a 24" 4k in the near future). Whether you will notice a difference, depends quite heavily on your own eyesight (and viewing distance, naturally). For me, even 1920x1080 on a 27" monitor is too much. Meanwhile for some people, even 2560x1440 on a 27" is too limiting.
Alternatively, you could get a 120Hz monitor, which lets you go over the standard 60fps, if needed, but still keep a steadier framerate on the more demanding games. And depending on your budget, you can now (since last month) even get a 120Hz VA monitor, as opposed to the cheaper 120/144Hz TN monitors.
Personally, assuming there's nothing majorly wrong with your 2407WFP, I would just stay with that for now. But if money is burning a hole in your pocket, then I would certainly go for the
Eizo FG2421.
Discussion of it in here:
http://forums.overclockers.co.uk/showthread.php?t=18553256
