Here's a review of the X800GT and X800GTO: Review...
Basically what the review explains is that the X800GTO has a better price/performance ratio and greatly outperforms the X800GT. Also if you're into unlocking pipelines, it says you may be able to unlock the last 4 pipelines to make it a X850GT. The specs of the X800GT compared to the X800GTO speak for themselves.