I have a 50"G10 in a theater room with no windows and no ambient light. I have been in the menu a lot since I got the display (about 2 months ago) trying to find the perfect picture. I have different settings for Custom, Game, & THX. I use THX for Blu-Ray's, Game for PS3/Xbox 360, and Custom for all other content. I will only be posting the Custom settings here...

Custom Settings

Contrast - 64
Brightness - 50
Color - 47
Tint - -1
Sharpness - 0
Color Temp - Warm1
Everything turned off
Black Level - Light

Never set your black level to Dark because it will crush all shadow detail and Sharpness is not needed on an HDTV. It is a remnant of old technology where noise was added to the edges of a picture to give it more detail.

You should not have to make any drastic changes to these settings even in a light room. At least these settings will give you a nice baseline to start with. If the picture appears too dark for you then slowly increase the contrast until the picture is bright enough for you. Increasing the contrast will make the picture brighter than increasing the Brightness.

Above all, enjoy your display. It is the best HDTV for the money and plasma is so superior to LCD. You made a great purchase.