I notice 3 temp. readings. The highest one is the one showing on Afterburner and Trixx and the lowest one is the one showing on cgminer and Kombustor. What should be the guide to know wheat the temp of the card actually is?
4 actually
93.0, 83.0, 93.5 and 91.5
I suspect that the GPU core temperature is the 1st one (93.0) and the other ones could be memory and the voltage regulator module.
If GPU-Z doesn't give any information beyond that and XFX themselves don't supply any additional software I'm afraid it won't be possible to find out with 100% certainty which reading is for which sensor.
Although these temperatures are a bit high (OK for the VRM and a bit high for GPU or VRAM for long-term use) they are still 'safe'. You won't make any profit with one HD 6770 anyway so unless you leave it mining 24/7 for many months I doubt anything should happen (although I guarantee nothing).