Rivatuner uses the same method of overclocking that the nVidia Control Panel uses. Basically on nVidia 6xxx series cards and higher there is a 2D mode and a 3D mode for GPU speeds. The RAM speed stays at the same speed for both 2D and 3D modes. So if you overclock the video RAM, that's where it stays all the time. When a 3D application starts, the GPU speed increases to whatever you set for the overclock level. I really don't know why they made only the GPU revert to default speeds in 2D mode. This may be different for the 8xxx series and they might have made both the RAM and GPU speeds speed up or step down in sync with 3D/2D modes.
I just tried overclocking my card but it just keeps going on with rivatuner. With nvidia's control panel I could maximum get a 550/1500. But with rivatuner I can go further, it just keeps allowing it. Is it safe?
The test settings button in Rivatuner may not work depending on your OS or video card. You should try stressing the card with a program like Rthdribl to find artifacts. I think the artifact tester in AtiTool works on nVidia cards as well.