well i have no experience using overclocking software with nvidia cards... but i use the radeon omega drivers
www.omegadrivers.net and they have nvidia ones too. im not sure how well they work.
you need overclocking software, which should come with omega drivers. however, dont just run into overclocking. first note the differences between the 6800gt and ultra.
1. 50mhz increase core.
2. 100mhz increase memory.
3. the ultra uses much more power, so the 6800gt at the same clock WILL NOT perform as well, so try to get the core o/ced 75mhz.
i normally dont suggest overclocking, but if you ARE using water cooling, depending on the cooler, you should be fine. try getting 3dmark03 or 3dmark05 and test to make sure that the card is running stable!