If your site has mainly static content and you're looking for a bit of a performance boost to tide you over until the next hardware upgrade/software rewrite, consider Varnish (
http://varnish.projects.linpro.no/ ). It's far more lightweight and efficient than something like Squid configured in reverse proxy mode. Just remember that Varnish won't work very well (or at all) if you don't send an Expires: header, or you set cookies.