alternatively goto
www.guru3d.com and download Rivatuner, relatively easy to use, and their temp readings are always spot on.....
6800GT should happily overclock to 410/1100 providing your case has good ventilation...
however it will shorten the lifespan a little bit, but not too much...
however its best to work up your clocks by 5 at a time, till the computer crashes (hehe) then back it off by 10-15 and settle for that...
you can go further if you take off the cooler on the card and buy an alternative more efficient one......however that carries dangers as if you screw it up you can lose the card, and it aint covered by warranty....BUT if it works will allow you to get your card up to around 430/440, possibly higher on the core, the memory mebbe 1150-1180....
oh, check your warranty, more than likely the overclocking will nullify your warranty...
so if it all goes tits up as they say in the trade you only got yourself to blame....shoulda bought the faster card to begin with...heh
(for how to use the rivatuner for overclocking etc, there are guides around, however if you search on AT forums for 6800GTO you should find a thread about what is it, its an old thread now, but if you look from around page 5 or 6 you will see my posts, eventually coming across a nicely detailed step by step guide to both unlocking pipes which you ignore, and then overclocking the core and memory.....should work the same for you as it did for me...)
edit: oh btw, never a wise idea to leave your card overclocked all the time....best to set the clocks a half hour before you go on a rampant gaming session...then when you know you finished set them back...it only takes a couple minutes if that.....altho you could leave them up permanently, it will definitely shortern the lifespan if your chip and memory were tested in the factory and considered not quite good enough to be an Ultra for sale......high clocks with a card like that wont do ah heck all difference for you in Windows, only in games....