from what i understand rivatuner will tell u the temps roughly, you can get this at
www.guru3d.com but some older drivers will display the temps in the driver options in the controll panel.
To give u an example my 7800 sits at 49c idle an it rises to 53c under load, i wouldnt expect a 6600gt to get any hotter than 55c.
But as ur intentin to try the card in another pc then that may be just as handy espically if there is no change in the way it acts.