RAM overload

I set up a new domain recently, and just this past weekend finally got some traffic. At this point, I went over my RAM limit because I had forgotten to set up Super-Cache on WP. I’ve managed to get in enough to activate the plugin, but the site is still going back and forth between extremely slow and 503/500s.

What does it take to reset the memory so that I can start over, this time with a cache?