LCD Monitors will usually have a Fixed or Native Resolution.  For instance 1920x1200.  Although other resolutions can be set, making the resolution lower on these kinds of screens will decrease sharpness, as an interpolation process is used to "fix" the non-native resolution input into the displays native resolution output.  When set higher than the physical screen resolution, some video drivers make the virtual screen scrollable over the physical screen or will result in a poorer image, due to dropping of pixels to make the image fit.

With the new LCD's available today, this perceived sharpness is usually not an issue unless you do fine detail work on pictures.  If you do, however, you may want to pay close attention to the Fixed/Native resolution of the monitor and base your decision on how big or small you are comfortable with see things on the screen when you go to buy.