I'm using RivaTuner 2.06 with an eVGA 7600GT and 163.75 drivers. I've changed the core and memory clocks, but the hardware monitor doesn't indicate that the changes are taking place. I've even stopped and restarted the monitor as well as RivaTuner. The sliders still show the new values, but the hardware monitor doesn't. The defaults are 560 / 700 I got to 650 / 750 which seems a little extreme but noticed no difference in HL2 demo. At some point the memory clock did show a higher value, but I don't know how or why it did, or went back to default.

make sure you are changing the clock speeds for the correct setting. theres 2d, and theres performance 3d and theres low power 3d

if your having trouble then you could set the performance to force the performance 3d clock speeds then just overclock the performance 3d clock speeds

If ure running vista then it wont change the valuse at all untill you dissable user account control and then reinstall rivatuner or you could flash the bios for the settings so they are on the card all of the time