Generally PHP memory limit set by root host is 8-32 MB But sometime you may need more PHP memory. If you are on a shared hosting, you can increase php memory limit of your account by simply adding the following code in your .htaccess file located in public_html folder.
[alert style=green_a] php_value memory_limit 128M [/alert]
This will increase php memory limit of your account to 128 MB. If there is no .htaccess file in public_html folder, create a text file and rename it to .htaccess