None, just put your money in an S&P 500 fund. I doubt you will be able to beat that in the long run anywhere else (Even though there are flaws w/ how the S&P 500 is run now [1]).
You can do better by diversifying more. By picking asset classes with a low correlation, you can reduce risk while increasing return. Hence, you can eek our more from a simple mix of say:
60% US
30% Int'l
6% REITs
4% Gold
All of this can be bought with low cost mutual funds.
60% US 30% Int'l 6% REITs 4% Gold
All of this can be bought with low cost mutual funds.
http://thismatter.com/money/investments/portfolios.htm