Mash potatoes with ricer or food mill over a large bowl. Stir in flour, sugar and salt just until mixed.
On lightly floured surface, divide dough into eighths. Roll each piece into cylinder 1/2 inch in diameter. With tines of fork, press indentations across the cylinder, down its length. Cut 1-inch pieces crosswise. Place dough pieces on baking sheets lined with waxed paper.
Heat marinara sauce in saucepan.
Meanwhile, bring large pot of water to boiling. Reduce heat. Immediately add gnocchi, working in 3 or 4 batches and dropping them in a few at a time to prevent sticking. Simmer until gnocchi just float to surface, about 2 minutes. Immediately remove with strainer or slotted spoon to colander; drain.
Spoon some marinara sauce over bottom of large serving platter. Add gnocchi. Top with some additional sauce; toss gently with rubber spatula. Sprinkle with cheese. Garnish with basil sprigs. Serve immediately with green beans.