Use Varnish, an HTTP accelerator. I use it on my Drupal sites. Haters can try DDoS all day and it doesn't matter. The webpages are cached and served from RAM, not disk. I forget the insane pages per second it can serve, but it's top notch. For example, Lullabot, a Drupal shop that runs the Grammy.com website for the night of the Grammys, use Varnish on it. It can handle insane traffic.