By outlawing the possession of gold, Roosevelt did take us off the gold standard domestically for all practical purposes.
Nixon simply closed the Gold exchange window to foreign governments. If he hadn't, Ft. Knox would have been cleaned out by European countries repatriating U.S. Marshall Plan dollars.
Two different gold standards. One is the classical standard, the other the ‘gold exchange standard’ IIRC. Both served to restrain inflation. It’s only after Nixon closed the gold window that the inflation of the 70s really took off. His option was put the economy into recession and reduce the growth of dollars, something JFK and LBJ had also been unwilling to do. The “Triffin Dilemma is what had confronted all three Presidents:
http://www.imf.org/external/np/exr/center/mm/eng/mm_sc_03.htm
Not really. FDR partially took us off the gold standard. The private possession of gold had nothing to do with the amount of gold that was backing the cash. He outlawed it to strengthen the dollar, which the depression was killing. Typical statist. Stick to your ideology (the government can control the value of the dollar) and destroy anything that disproves that theory (commodities).