I actually have wp-supercache enabled. However, the site was only running on 512MB ram :( Getting featured on HN is a lot different than daily traffic. A static site would not have saved me here.
Wordpress cacheing HN traffic should do fine too. There's something else going on here.
HN traffic isn't really that much. I was watching the stats under the new Google Analytics realtime mode and the site was averaging about 3 - 6 requests per second.