trouble forming xpath expression with compound conditi

With the xml given below, the context node it <items>.
I want to select the <item> elemesnt for which the attribute
@id='3' and <si> content is SI7 (this would be the second <item>
instance)? The desired
attribute value
would be stored in a variable $myid and the desired content
would be stored in a variable $mysi. Anyone know how to form
the xpath expression within <xsl-value-of select="xpath expression"/>?

<items>
  <item>
     <si id="1">SI1</si>
     <si id="2">SI2</si>
     <si id="3">SI3</si>
     <si id="4">SI4</si>
  </item>
  <item>
     <si id="1">SI5</si>
     <si id="2">SI6</si>
     <si id="3">SI7</si>
     <si id="4">SI8</si>
  </item>
<items>

_________________________________________________________________
Get your FREE download of MSN Explorer at http://explorer.msn.com/intl.asp