Io non ho capito ma provo ad indovinare...

<select onchange="window.open(this.options[this.selectedIndex].value,'finestrella')">
<option value="http://www.html.it">html</option>
<option value="http://www.google.it">google</option>
</select>
<iframe name="finestrella" width="400" height="300"></iframe>