Stocks to Watch Monday  British advertising firm WPP &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=WPP.L target=/stocks/quickinfo/fullquote"&gt;WPP.L&lt;/A&gt; has agreed to buy  smaller U.S. rival Gray Global for \$1.31 billion, equally  divided in cash and shares, to put it on par with industry  leader Omnicom &lt;A HREF="http://www.investor.reuters.com/FullQuote.aspx?ticker=OMC.N target=/stocks/quickinfo/fullquote"&gt;OMC.N&lt;/A&gt;. Gray Global shares closed at \$940 on  Nasdaq.  