I have created a custom button to create a New Opportunity and I'd like to pass in the Account Name, but am not sure how.  Here is the code I have in my visualforce page.  I think I need to add additional paramaters to the !URLFOR part, but if someone could help me out I'd appreciate it!  Thanks.

<apex:page standardController="Account" extensions="AccountExt" sidebar="false" showHeader="false">
    <script>
      function doRedirect()
      {
       window.parent.location.href = '{!URLFOR($Action.Opportunity.New)}';
     }
    </script>
    <apex:form >
    <apex:pageBlock title="Opportunities">
        <apex:pageBlockButtons location="top">
            <apex:commandbutton onClick="doRedirect()" value="New Opportunity"/>
        </apex:pageBlockButtons>
        <apex:pageBlockTable value="{!AllOpportunities}" var="item">
            <apex:column headervalue="Opportunity Name">
                <apex:outputLink value="/{!item.Id}" target="_parent"><apex:outputField value="{!item.Name}" /></apex:outputLink>
            </apex:column>
            <apex:column value="{!item.StageName}" />               
            <apex:column value="{!item.Amount}" />
            <apex:column value="{!item.CloseDate}" />
            <apex:column value="{!item.OwnerId}" />
        </apex:pageBlockTable>
    </apex:pageBlock>  
    </apex:form>
</apex:page>