It uses about 7GB for a basic install. My CPU is about 90% idle when the machine isn't (visibly) doing anything. With Safari, Mail and Activity Monitor running the system is showing just over a gig of RAM in use, but only about 585MB of that is active (not sure what the rest is doing - just allocated for the system I suppose?)
It is *very* responsive. Certain things actually feel faster than Tiger. The new Finder is lovely. I've overcome some of my gripes with the new interface, namely getting the horrible 3D dock to revert back to 2D. So now I am happy :-)
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moto