If you are comparing frame rates between different games like Crysis, Far Cry 2, Fallout 3, Left 4 Dead... They all use different game engines which stresses the video card differently. Those games are listed in order of increasing performance.
Crysis crams in as much video effects as possible (some people also suggest poor coding). Left 4 Dead is based on the aging engine used by Half Life 2 which has good graphics and did not overly stress the video card back when it was released.