magento setup script for core config data
<?php
$installer = $this;
$installer->startSetup();
$setup = new Mage_Core_Model_Config();
$setup->saveConfig('catalog/frontend/grid_per_page_values', '8,16,24', 'default', 0);
$setup->saveConfig('catalog/frontend/grid_per_page', '8', 'default', 0);
$installer->endSetup();