Dell's 6800U's and 7800's clock speeds are locked by the BIOS, changing them in the drivers has no effect. The 6800 non-Ultra doesn't have this limitation which is why it can be overclocked via the drivers. For 6800Ultra and 7800 owners, the only way to overclock is with modified BIOS's which you can read about here:
6800U: http://www.notebookforums.com/showthread.php?t=92401
7800GTX: http://www.notebookforums.com/showthread.php?t=117648
6800U: http://www.notebookforums.com/showthread.php?t=92401
7800GTX: http://www.notebookforums.com/showthread.php?t=117648





