'Except', not 'expect'. FYI
Is your 20" widescreen?
Can you run the game in 'windowed' mode? Because it sounds like your monitor's acceptable resolutions may not be compatible with the game's expected resolutions. I doubt that the refresh rate is the deciding factor, because LCD's don't usually go very high with the refresh rate. You are using a newer generation of monitor than the game has capability for.
GalCiv2 has the option (in the Options/Video screen) of running either in fullscreen or windowed mode. If this game has that option, try the windowed mode. If it doesn't, well, it simply may not run on your newer hardware.
You could also try running the game in 'Compatibility mode' to an earlier version of Windows. (right-click the launch icon and choose Properties. Then select the 'Compatibility' tab and set it for another version)