Stocks Gain on Siebel, Oil Below \$50  NEW YORK (Reuters) - U.S. stocks rose on Monday as software  company Siebel Systems Inc. &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=SEBL.O target=/stocks/quickinfo/fullquote"&gt;SEBL.O&lt;/A&gt; boosted technology shares  with a better-than-expected sales outlook and Friday's  start-of-quarter buying spree ran into the new week.