Did you try Call of Duty 4 demo? You should before buy it
The Engine is pratically the same of CoD2 and the gameplay is really similar...so if you liked it you'll enjoy this
...but nothing new. You won't have problems running it, because graphically is not so "heavy".
About gameplay you'll see new things because this is a modern war game (In CoD2 you play the WWII).
Maybe The new Medal of Honor is better...
I can't compare Crysis with CoD4, it's two steps forward...