Running in windowed mode
If you have an LCD monitor with a native res that is higher than the res at which you want to run the game (e.g. 1680x1050) and you run the game at lower resolutions e.g. 720p, it will look poor due to the monitor doing all kinds of crazy scaling. Thats the downside of using some cheap LCD monitors which usually have terrible scalers. Hence the reason why running games at and LCD monitor's default resolution is always best. But then again depending on the game, you need a seriously good rig to be able to do that due to the native res for some monitors being quite high.
CRT monitors do not have this problem.
Though unsupported, you can add -w to the game's shortcut to run in windowed mode and see if that helps.