Ok this is the topic that I used to overclock my graphics card (Leadtek 6800nu 128mb). This involved modding the graphics card bios and upping the voltage and relaxing the ram memory timings to achieve higher core and memory speeds. Here is the link:
http://forums.guru3d.com/showthread.php?s=&threadid=133245&perpage=25&highlight=&pagenumber=1
I can not be held responsible if you try to modify your graphics card in any way.
This did work for me as I went from the stock speeds of 325/700 to 425/950 and I am lucky that I could use Rivatuner to open the extra pipelines and vertex shader. Therefore I have an oced 6800 Ultra core with 128mb of DDR1 ram. Free increase in performance, but shorter life span, but I don't plan on keeping this card for too long. Please don't try this is your not confident with it.
!TIMMY!