Money can't buy happiness... but not having to worry about money buys less stress.
Norway and United States have very similar GDP per capita. The important difference is that in Nordic countries the money has been divided more equally.
Judging by the top entries in this list, GDP PP is a pretty good happiness predictor.
There's some good data on this. I seem to remember happiness responding to income up to about $80k/year and the curve continuing rather flat above that sum.