I would go with the x800xl, it's cheaper than the 6800GT and will do just as well. Still, if you want to start with settings up much higher as posted earlier you'll need to do a lot more than this.
While the x800xl is a great price it doesn't overclock well. Some 6800GTs will overclock for you, but not to the level you want. I say get the x800xl. You're upgrading for the exact reason I did, and this is also the card I upgraded to. You should know that the x800xl doesn't give me even close to what I want out of BF2, but I don't think I'm going to get that until I upgrade my CPU considerably.