are you raising the stock 1.5v that's set in the BIOS? Because that doesn't correlate with the actual GPU Vcore (which could be thought of like the CPU Vcore)
A stock 6800GT runs at 1.4v, the power going to the AGP slot as set in the BIOS is 1.5v....I've seen some people say that helps in higher overclocks, some say it's BS...personally never tried it myself.
However, you can get a program like "Nibitor" and it'll allow you to open up and change your video cards BIOS to a higher voltage.
Flashing it with an Ultra BIOS would also do the same, but I heard that the GT's RAM timings are supposed to be faster...no you can't change the RAM timings, there are options in the video cards BIOS when looking at it in nibitor, but it's all like....hexadecimal lol, soooo yeah theres no sense in bothering with it.
HOWEVER, I've heard of a lot of instances of people raising the Vcore through their BIOS or even using an Ultra BIOS and it causes their card to burn out after a couple months
85C was the hottest my card ran when it had stock cooling as well..and that was at ultra speeds.