Visibility Issues with Mail and iCal in OS X Lion
It seems like the contrast on your display is too low for you. If you can adjust the contrast on your monitor, try increasing it. If you cannot change the contrast (such as the internal laptop displays), fuss around in the color calibration utility (go to System Preferences > Displays > choose the window on the monitor you want to change > Color tab > Calibrate… and follow the instructions). Make sure Mail and iCal are visible behind the calibration window so you can see if your changes are sufficient. I'd suggest using Expert Mode and paying especial attention to the vertical sliders on the left.

To answer your final question regarding resolution, upgrading to a higher-res machine would not help. Apple's machines are usually fairly standardly calibrated from the factory, so a higher resolution would simply make the pixels smaller, not easier to distinguish, and might even make things worse.