So you are asking several questions.
1) how bright.
The standard a few years back and still a good benchmark is 60 lumens. But with modern LED technology 200 lumens is now the norm for under $100
2) stand alone versus weapon mounted
This goes more into preference and I do personally and professional recommend a stand alone dedicated light. This gives you the flexibility to use your light without drawing your firearm.
3) technique
How you hold is up to you. There are lots of different ways to Hold High or Off to one side but if you deploy a firearm you have lost a great deal in support in firing. If you are good at one handed shooting...
Just note that lumens alone is not enough. Each light is different and the quality of the LED differs too. You pay for what you get... Don't get me wrong as even cheap lights are pretty darn bright these days.
If you are in the
Seattle area - September 24, 2011
Check out this:
http://waguns.org/viewtopic.php?f=60&t=2130