i would just calibrate your TV yourself though, then you wont need to use it(just pick up a calibration disc) or follow the first link in my sig
cause if you dont calibrate your HDTV your only getting about 70% picture quality
"Game Mode" reduces the amount of processing involved in producing the image on the screen.
This speeds up the image and reduces input lag which can be very important in high-speed high-intensity videogames.
If you want to read about input lag go here: http://en.wikipedia.org/wiki/Input_lag
Basically, it means that there is delay from the moment the video leaves the PS3 to the mement it reaches your eye.
"Game Mode" is intended to reduce that time.