Some companies try to pass of NV41's and HSI bridged NV42's as 6800GS'. So if you have the NV41, sit back and cry...your card wont unlock, and wont overclock that well. NV42, wont unlock, but will overclock very nicely to compensate for the lack of pipelines.
Wusy is right, dl Rivatuner. Confirm you 6800 being NV40.
Then on the customize tab in RT, select that and go to the Nv strap at the top.
Go to the bottom of the opened window and click on the install button.
It should give you the option to support for unlocking and advanced hardware.
Click on the enable masked hardware units...then a little warning should pop up somewhere. Click yes if you want to continue....then select 'custom' from the text and you should have a drop down box that will give you pixel and vertex unit configuration.
Quite easy to see, you'll see several bits that are disabled and masked.
Just check off the box next to them to enable the units. click ok then apply.
Reboot, hopefully no artifacts should appear, because then your pipes were damaged from the factory
If that should occur, you can uninstall then reinstall riva tuner. Go through individually and check off your units, leaving you with your defective ones flagged disabled and masked, but still able to use the other functioning ones.
I was in a little rush when I posted this, but just a decent amount of common sense and you'll see exactly what I'm talking about. Good luck.