I think this might be what you're looking for:
<xp:button value="Click here to open Google in a new window" id="button1">
<xp:eventHandler event="onclick"
submit="true" refreshMode="norefresh">
<xp:this.script><![CDATA[window.open("http://www.google.com", "_blank");]]></xp:this.script>
</xp:eventHandler>
</xp:button>
Of course, you'll need to change the CSJS (or you could do it with SSJS) to open your xpage vs. url.
Hope this helps!