I cant see it running good at all with a 6800GT.
I just tried this game, and I get bad lag, down to 15fps most of time, I have turned Grass Shadows off, Sun Shadows, no AA or AF, went to Nvidia Control Panel, got 8xAA and 8x AF, 1440x900 res, werid thing is, my ram was only at 86% and the other 14% was not in use by anything including windows, and the CPU Usage kept jumping around, 50% - 80% - 100%, then stayed stable at 80% at one point, I find this weird because I would have thought the CPU should have been at a constant 100% which it is in all of my other games concidering I have a single core CPU.
On S.T.A.L.K.E.R. MP I went into a server by myself, and I had it maxed out at 1440x900, 16xAF 16xQ AA, and everything except grass shadows and sub shadows turned on, lowest FPS on avarage was about 30fps, max about 100.
I tried it again, this time ram was at 99%, CPU never was at 100%, more like 40% sometimes, something tells me its not using all the CPU power, but I can see 1gb more ram helping me, and getting a 3800+.