GM Hikes Incentives After Weak Sales  DETROIT (Reuters) - General Motors Corp. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=GM.N target=/stocks/quickinfo/fullquote"&gt;GM.N&lt;/A&gt; on  Wednesday raised its cash incentives by between \$500 and \$1,000  on most of its 2005 model year vehicles, a week after company  officials expressed disappointment with August sales.