Memory Exhausted after Drush install
Note that Drush and Drupal use different php.ini files.  This behavior is inherited from php, which allows for different configuration for commandline tools vs. web servers, so that your web server can be more locked down than your dev tools.

Use the drush status command to see the path to the php.ini file that Drush is using.

Use the drush status-report command to see the commandline equivalent of the reports -> site status page.  This will show you the real memory limit that is set for Drush.

Odds are, all you will need to do is change the memory limit setting in the php.ini file that Drush is using, and all will be well.