Here's a list of screen resolutions :
http://www.ngohq.com/peripheral-equi...ions-list.html
The higher the resolution the more powerful the graphics card needs to be to run games at an acceptable frame rate.
Throw in AA and AF and the power requirement goes up higher.
Add in a game like Crysis ( and enable soft shadows while you're at it ) and you can bring any system to its knees with high resolutions and all the extras turned on.
So why do you want to turn everything up to high?
Because games can look so much better that way, provided you can still run them.
If you have an lcd you know about using native resolution.
With crt monitors you could change resolutions to get yourself decent frame rates if the game was pushing your graphics card too hard or if you just had to have AA and soft shadows enabled.