nforce, I'm not exactly sure what you're saying but you don't have to have both monitors running the same resolution nor is it any better (other than some continuity) to have two identical monitors, IMO.
Skell13, I'm guessing you are running both monitors at the same time through the GTS 250. What model is your 24" monitor? When you go to your screen resolution property, are you able to change the settings for both monitors? If you're only running the 24", what is the highest possible resolution? Usually, if a 24" monitor supports 1920x1080, it'll automatically set it as the default resolution.
Also, try downloading and re/installing the latest drivers from www.nvidia.com .