There's a patent for storing a specific database structure in memory and therefore MySQL Server memory tables use a hash structure to avoid this patent. This would very well when you lookup a row which is present. However, the entire hash is enumerated when an row is not present. So, perversely, you'd get more performance with MyISAM, a RAID card and a big disk buffer.

You could use memcached. However, if you are developing REST web services, you can retrospectively introduce Squid without changing your code.