Using FTP, try increasing the memory for PHP and Wordpress by editing the memory_limit line in your php.ini (if you have access to it) to 64M or 128M, which should be fine for non-multisite installs.
memory_limit = 64M;
If you can't get to the php.ini file, add this line at the top of your .htaccess file:
php_value memory_limit 64M
If that doesn't work or throws an error, add this line near the top of your wp-config.php file right after the opening
But your host may limit changes to .htaccess in your account and to the host-wide php.ini, so these may not work. if not, ask your host.
You can run phpinfo to see what kind of memory allocation you already have. Make a plain text file and call it phpinfo.php and put just this line in it:
<?php phpinfo(); ?>
then put the file in the root of your server, and then go to that file with your web browser. You'l see all the php info you need; scroll down for memory settings. If you get an error, that means your server company won't allow phpinfo to run for security reasons.