Also released with Version 2.5, the WP_MEMORY_LIMIT option allows you to specify the maximum amount of memory that can be consumed by PHP. This setting may be necessary in the event you receive a message such as “Allowed memory size of xxxxxx bytes exhausted”.
This setting increases PHP Memory only for WordPress, not other applications. By default, WordPress will attempt to increase memory allocated to PHP to 32MB (code is at beginning of wp-settings.php), so the setting in wp-config.php should reflect something higher than 32MB.
Please note, this setting may not work if your host does not allow for increasing the PHP memory limit–in that event, contact your host to increase the PHP memory limit. Also, note that many hosts set the PHP limit at 8MB.
Increase PHP Memory to 64MB
define('WP_MEMORY_LIMIT', '64M');
Increase PHP Memory to 96MB
define('WP_MEMORY_LIMIT', '96M');
thanks http://codex.wordpress.org/Editing_wp-config.php#Increasing_memory_allocated_to_PHP