I find that auto detect overclocks just a little bit too much, my 6800gt runs stanle at 410/1130, auto detect ran it at 415/1150
Ati tool is a good one to use also to test for artifiacts - but I wouldn recommend using that to find max overclock as that found like 420 on my core and 1160 on my memory - and even at that it was showing a little white box that the program wudnt pick up as a artifiacts, so i moved them down slowly 10 by 10 until that little black and white dot disappeared, and it did so at 410/1130 to be 100% stable.
I could probably run games at 420/1150 and not get artifiacts but I know it wouldnt be 100% stable.