I checked with Real-Time HDR IBL, and temps never exceeded 61 C degrees.
I think for my NX 7800GT this is a quite normal temperature. I used different
settings, even higher values such as 16X multisample, higher depth of field
values and also the highest resolution of 1280x1024, in every configuration
the GPU never exceeded 61 C degrees (by the way rthdribl is a great application!!).
I also tested my rig with prime 95 for 8 hours without reporting any errors.
To sum up,
Best settings for my rig are: 2-2-2-5-1T, vdim:2,80v, preamble value:5ns.
At the above settings all the applications completed successfully without any errors (prime 95, memtest86, super pi mod 1.4, S&M).
When I lowered vdim at 2.75v prime 95 and S&M reported errors while super pi mod 1.4 and memtest never gave me a trouble.
At last I would like to thank all the people who helped me make my rig running flawlessly.
Thanks, thanks, thanks!!!!!!