rerendering is always a bit tricky - at least if you want to rerender forms. This is a lection I've learned by using richfaces 3.x. I never render forms. Only a4j:outputPanels or h:panelGroups inside forms.
The <f:ajax> doesn't have an onclick attribute - as far as I know.. "onevent" I think.
You cound try to only rerender the parts of your page which actually need do be rerendered.