Depends on what kind of lag your talking about.. Lag is really from your internet connection when you play multiplayer servers... My system runs f.e.a.r really smoothly I have a 3ghz cpu and 1.5 gb of ram with a 6800 gt oc to 6800 ultra.
The 7800 gt is nice but just abit more and you can get a 7800 gtx. The game will run smooth with that amd dual core processor yes. 2gb of ram I recommend.
srry, I ment to ask if the 4400+ and 1gig of ram was enough to run battlefield 2, FEAR, CoD2 smoothly cause I dont think I can get 2 gigs right now