Cadre Performance Pack - $25/mo
Varnish is a HTTP accelerator that caches web pages for future use. When the same page is requested by a user, it is provided by Varnish instead of traversing the LAMP stack. Since the request never reaches the LAMP stack, this significantly decreases the page load time and the load on the server. It should be noted that Varnish is generally only useful with anonymous traffic.
Memcache is a powerful caching daemon designed to store pieces of data such as objects and strings. It is most useful from a hosting standpoint in that it can store results from database queries. This reduces the amount of calls needed in the future, thus decreasing code execution times.
APC "Alternatice PHP Cache" is an opcode cache used for optimizing PHP intermediate code. This service reduces memory consumption used parsing and compiling the code each time the page is loaded by caching the compiled form of the PHP scripts.
Solr is a search platform that uses a variety of features to speed up complicated databases queries. While Solr can drastically reduce the amount of time required for queries in large pools of data, it should only be applied for sites where the increased memory usage doesn't outweigh the benefits.
and a 1GB RAM upgrade!