I use a Sapphire Radeon 4650 1 GB DDR2 .....It's been having some overheating issues lately. First the fan stopped working, but then I dusted it & now it works but gpu temp is still very high at zero load. BTW, I overclocked it long ago with auto-tune function of amd catalyst. It had no issues back then. After the overheating issues started showing up, I changed it back to factory settings. I have the latest driver btw.
If that's your idle temp and you're sure the fan is now working, then I think of two possibilities:
-Somewhat likely: Your cooler has somehow dislodged itself from the GPU, and is not getting appropriate heat transfer from the chip to the heatsink.
-Less likely: The temperature sensor is busted and giving you faulty readings.
If you have access to another PC you could try that card in it and see if you get high temps. Also your card could just be on its way out. But 70c at idle is way to hot needs to be around 35c avg. Good luck to you.