As long as your CPU can keep up with the game resolution will almost always be the biggest factor. For LCD monitors the native resolution should always be used if at all possible though imo so it is other settings you will want to lose or lower to improve performance. AA is often very strenuous and especially at higher resolutions it should be the first thing to be turned down/off. Shadows are another thing that can often be turned down slightly and have a large increase in performance.
That is just some general guidelines. As ct says it really depends on the game. This site has some (very) in depth guides to the graphical settings of a lot of the most popular games over the years;http://www.tweakguides.com/Games.html
I haven't noticed them doing any new games for a while unfortunately but looking them over will still give you a lot of knowledge about game settings in general. Their articles on driver settings are good stuff as well.