I too have been OCing my 8800gt using riva tuner. However i have found some issues with the clock speeds (and memory). Its most pronounced on the shader clock speed- I have found that when i increase the speed a little (say 10mhz) it registers on GPUZ as a 10mhz increase, but on riva tuner it doesnt show up. The default OC of my card is 1620 on the shader. The only OC's that riva tuner registers are 1678, 1728 and 1780 odd. Any small steps dont register, but they do with GPUZ.
Eg when i set it at 1720 it still says 1678 on riva tuner....when i bump it up to say 1725 its suddenly goes to 1728.
Riva tuner is def telling the truth (not gpuz) as a can run ati tool art canner at 1720 and have no problems ...as soon as i go up to 1725 the odd artifact is detected.
Just wondering if this is this normal?
Ive currently settled on 690/1678/955 as my OC