both. I also have 1440x900 native resolution. I don't play a lot of fps, but I did benchmark the crysis demo extensively with my 3870. I get around 26 fps on all high with no AA in winxp, all clocks listed in my sig. Based on what I've read, I would get well over 30 fps with an 8800gt, probably closer to 35. My oc isn't bad at 860/2602, but I've seen a LOT of people post 20+% oc's on their evga stock cards. With that kind of oc you should be able to get 40 fps on all high as long as you have a nice c2d or better and 2gb ram or better. You could probably even throw in some AA and still get over 30 fps, a result that many gamers of today would kill to get.
I didn't get my card for crysis and I realize that is just one game, it's just that all of the other games that you mentioned will run great on either card at very high quality settings. now, admittedly, ati cards don't do very well in crysis right now, but 8800gt seems to do better with AA than 3870.