Richimself, I used the defult video setting and fraps for this test and I found that I have frame rates from 70fps with little happening to 55fps when in a huge battle with many other players around. I have some setting turned up when I normaly play. and I seem to have stable fps around 55-60fps, which I find suitable.
I suggest when you build your system you go with the 6800gt, if you have the money for it. You will be able to play BF2 with higher settings and enjoy its great graphics.