The default settings are usually pretty good. However, I have always used the recommendation of Koroush Ghazi on Tweak Guides. http://www.tweakguides.com/NVFORCE_6.html
The settings are a few years old because he has been busy writing game guides for Gforce and doesn't update his site very often.
These are the global settings I use for my 2 x GTX 570s.
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