As an x850xt owner, I can say that i'm very satasfied with my video card. Excellent quality on all my games. I also get some A$$-whoopin' scores in 3D Mark 05. (~ 6200 3D Marks)
I know that it doesn't support PS3, but by the time a good game that uses PS3 comes out, I'll be ready to upgrade. Whats another $500-$600 for something that gives me so much pleasure. *hugs computer*
And about your Question, I would go for the X850XT. (if you're going to go with ATI.)
Obviously the 6800GT is a great card also......