Congress can keep the systemic risk posed by Fannie Mae and Freddie Mac in check by creating guidelines on the appropriate level of investment portfolios for the two government-sponsored enterprises, according to Federal Reserve Board Chairman Alan Greenspan. In testimony to the Senate Banking, Housing and Urban Affairs Committee, Greenspan advocated that a $100 billion to $200 billion cap on the GSEs’ portfolios be phased in over time. He added, however, that he…