When using XPath bindings in XAML, how can I use a variable in the path?

If I have

<TextBox Name="MyInput" />    
<TextBox Name="Result" Text="{Binding XPath=//Item[@Id='{X}'/Value}" />

Where “X” is replaced by whatever is entered in “MyInput”. Attempted a mulitibinding, but that only resulted in the actual binding statement being displayed.

<TextBox>
    <TextBox.Text>
        <MultiBinding>
            <MultiBinding.StringFormat>
                <![CDATA[//Item[@Id='{0}'/Value]]>
            </MultiBinding.StringFormat>
            <Binding ElementName="MyInput" Mode="OneWay" Path="Text"/>
        </MultiBinding>
    </TextBox.Text>
</TextBox>