A downloadable software called AIDA can measure temps of ur hardware eg. proc, graphic, HDD etc.. Another one is hwmonitor. The Hottest part in laptop would b the GPU (graphic), better keep it lower than 60C or ur room will suffer from green house effect lol. A good laptop cooler will dissipate the heat generated out off the laptop, not blowing air onto the laptop.

Its been discussed here: http://forum.lowyat.net/topic/511950
Aluminium based is good. Btw i've never used laptop cooler, coz i dont like the vibration it makes it is damn annoying.