On many web hosts, PHP is configured by default with a memory limit of 8M. While the minimum installation of Drupal will work with that, once you start adding a lot of other modules, like Ubercart, you will run into a problem. Ubercart/Drupal will need a minimum of around 12M (for nothing but the core modules and its dependencies) to a maximum around 18M (for all Ubercart modules enabled); you might have to try 24M or even 32M if you plan on using more Drupal or Ubercart contrib modules.
The solution is to increase PHP's memory limit in one of these three ways, in order of preference:
- Add the line memory_limit = 24M to your php.ini file (A value of -1 can be used instead, meaning "no limit")
- Add the line ini_set('memory_limit', '24M'); to your sites/default/settings.php file
- Add the line php_value memory_limit 24M to your .htaccess file in the Drupal root directory
Only one of these is needed. If you have problems making this work, your web host provider should be able to help.