If you want to open the (existing) pages in a popup window when a selection is made you can use code like this:

<apex:page>
    <apex:form>
        <apex:selectList size="1" onchange="return selectChange(this);">
            <apex:selectOption itemLabel="--None--" itemValue=""/>
            <apex:selectOption itemLabel="ABC" itemValue="/apex/A?id=123"/>
            <apex:selectOption itemLabel="XYZ" itemValue="/apex/A?id=456"/>
        </apex:selectList>
    </apex:form>
<script>
function selectChange(select) {
    window.open(select.value, '_blank');
    return false;
}
</script>
</apex:page>

where instead of the above hard coded apex:selectOptions you would create then in your apex:repeat loop.

If you want to open the (existing) pages in a popup window when a selection is made you can use code like this:

<apex:page>
    <apex:form>
        <apex:selectList size="1" onchange="selectChange(this);">
            <apex:selectOption itemLabel="--None--" itemValue=""/>
            <apex:selectOption itemLabel="ABC" itemValue="/apex/A?id=123"/>
            <apex:selectOption itemLabel="XYZ" itemValue="/apex/A?id=456"/>
        </apex:selectList>
    </apex:form>
<script>
function selectChange(select) {
    window.open(select.value, '_blank');
}
</script>
</apex:page>

where instead of the above hard coded apex:selectOptions you would create then in your apex:repeat loop.