NVIDIA cards now have a fairly decent driver support - including temperatures and fan sensors. (nvidia-settings and nvidia-smi, e.g.
nvidia-smi -a | egrep -w "C|Used|Fan"
^--this command line works for me. You can wrap it into an alias.)
But for ATI cards (don't have them neither at home nor at work), well, there are probably equivalents - you may want to look for them. There's also lm-sensors.org ...
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