Wal-Mart Sales Estimate Boosts Sector  NEW YORK (Reuters) - Wal-Mart Stores Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WMT.N target=/stocks/quickinfo/fullquote"&gt;WMT.N&lt;/A&gt;, the  world's No. 1 retailer, on Monday said December sales would be  at the top end of its prior estimate on better-than-expected  post-Christmas spending, driving its shares up 2 percent.