How do I check my VRM Temps on ATI 4850?

Assuming you have the same GPU-Zas me you have three temps at the bottom of the sensor page.
They are GPU then Ram then Shaders.
Do use other apps to get a better idea as one app alone may give false readings, its best to run a couple at least to check.
Rivatunner as said or even the CCC itself will give you an idea Si Soft Sandra Lite is ok as well.

Mactronix