$terms = Mage::getResourceModel('catalogsearch/query_collection')
                ->setPopularQueryFilter(Mage::app()->getStore()->getId())
                ->addAttributeToFilter('query_text', array('like' => strtolower($this->getRequest()->getParam('letter')).'%'))
                ->setOrder('popularity', 'DESC')
                ->setPageSize(100)
                ->load()
                ->getItems();