I think you should go for a 6800GS, and Im sure that will run any games yo want on medium/high settings.
Also I think the 6800XT may have slower clocks than the GS, I have seen a XFX XT with 350/950 clocks but my gs (cheap model {Inno3D}) clocks are 425/1000 which can easily be oc'd
Altogether I do think the GS would be a better buy that is if you have the money for it!