(Home prices article updated with homebuilder stock movement and additional commentary.)NEW YORK ( TheStreet) -- Stocks in the homebuilder sector moved higher Tuesday morning after a better-than-expected report from the S&P/Case-Shiller 20-city index of national home prices. The index rose 4.4% in the second quarter, after a 2.8% drop in the first quarter. Economists had expected the index to rise just 3.1%, after rising 4.6% in May. A federal tax credit for homebuyers of up to $8,000 is largely credited with strengthening home sales this past spring as buyers rushed to push up their purchases before the credits expired April 30.