from personal experience with Crysis and COD 4: Crysis
Haven't played the upcoming games. I'm guessing that Farcry 2 will be using the 'optimized' engine that Crysis:Warhead uses.
This advantage may be negated if there are more items / more detailed scenery in the game.
Far Cry 2 uses a totally different engine called Dunia.. it's developped and published by Ubisoft.
Crytek developped Crysis (newer Budapest studio made Crysis Warhead) and is published by EA.
That's good to know Rob, that FC2 won't kill my Q6600 and 9800GTX.