Better site and database performance
1 - Use captchas and spam filtering; Mollom is a good place to start. There is also  info on hardening Drupal in the drupal handbook, ie. http://drupal.org/writing-secure-code.

2 - For starters, turn on the performance module that comes with Drupal.

3 - This is a big question. I have heard good things about Context HTTP Headers, a module contributed by the White House Drupal team. You can find a comparison table on Caching: Moduls that make Drupal scale.