Does profit equal increased wealth or merely reshuffled wealth?
To an empirical degree we can certainly state, that in at least currency terms, all imports drain wealth from a country (unless currency value degrades faster then what is bought...). Yet we print money fast enough that we don't run out of it. As we print it, inflation occurs, and dollar reserves around the world go down in value, neccesitating fresh aquisitions out of the American homeland. The US is rich because of dollar exportation (oil can only be bought with dollars, and thats why they are needed), though I suppose thats besides the point...
What is the most essential internal component to a strong economy?
My best guess would be proper circulation.
Around 90% of bank saved money is used to give out loans; that is to say, it is invested. Thereby "saved money" is really invested money (as long as it is placed in a vault, as opposed to a sofa).
The best economy will be that which balances "saving" and direct investment, with spending.
The Middle class invests and spends, and so does the upper class. The poor can only spend, as their subsistence needs don't leave them with enough excess to save.
In order to know which class is the most economically beneficial to the economy from a spending standpoint, you would need to know:
1. What is the precise ratio of best balance when looking at investment and spending.
2. Which classes spending and saving habits most closely meet 1's ratio. If the balance is to far towards the investment side, then the best economy would have the poor to subsidize the spending size of the equation.
Extrapolating from this theory of economics I just made up

, I think we can state, that at some points of time, when far too little spending is going on, money may actually be most valuable in the hands of the government. If no one is spending privately, the government can forcedly take up the slack, and while it may not spend in the best areas, at least it will spend. This would explain why the new deal was helpful to us, though the uplifting of the US economy during WW2 was I believe a result of spirit making uneccessary material incentive.
Macura:Investments will never reap anything without a market; that is without buyers.
Also, remember that all banked money is being invested via loan.
That having been said, the rich do more direct investing then the middle class does, which may be good or may be bad. Are they perhaps spending too little? They obviously didn't get to where they are via an ireverence to making their fortunes self multiply.