You could check the PCB for monitoring chips, if you're up to it. It's possible you're just stuck with a basic video card without any GPU/fan monitoring abilities.
If you haven't done this already: Make sure you enable all the plugins Rivatuner has. With the monitoring window open, click on Setup in the bottom right. In the Hardware monitoring setup window, click on Plugins, at the bottom left. In the Active plugin modules window put a checkmark next to all of the .dlls and click Ok. With any luck you might see your GPU temps and fan speeds.
If not, try some other programs like: Ati Tray Tools, Everest, Speedfan, Furmark, HWMonitor, and HWInfo32.
Good luck!