1. Install and enable
mod_pagespeed (http://code.google.com/speed/page-speed/docs/module.html) plugin in Apache. What the plugin does it that is automatically
applied the best web practices to web pages. For example it a) minifies
external css and javascript (compresses); b) combines external js into a single
file; c) optimizes cache (sets proper expire http headers for static resource);
d) uses url-fingerprinting when combining files; and e) minifies HTML

2. Consider using Nginx as a main server. Nginx is much faster, resource aware compared to Apache. Apache is this gigantic beast running in the background which can handle pretty much
anything, however a lot of those capabilities are not necessary. When Apache
gets a lot of requests, it will spawn (create) a separate process to handle each
request. This has two side-effects: a) eats a lot of ram so in the future fewer
requests can be handled and b) since your cpu has only a certain amount of
cores, for os it is much harder to handle many cpu intensive processes at once.
So when apache spawns processes to handle each request, it in turn slows down
the computer. On the other hand, Nginx has a concept of workers. you can define
how many workers will run at the same time. Usually that would be the same
number as the number of cores you have available. Then when Nginx will receive
requests, it will forward them to one of the workers. When Nginx will receive
many requests, the workers are basically going to handle all the requests as
fast as they can. This makes the whole operation much faster compared to
Apache.

3. Consider running Nginx as
reverse proxy for load balancing. You can get additional dedicated servers, so
then Nginx as it will get requests from users, it will just forward them to
other machines. The other machines can run anything – Apache, Nginx or any
other server.

4. I assume that you are
using MySQL as a database. Switching to something like Postgresql can provide a
pretty big performance boost.

5. Regularly optimize
databases. When you insert or delete a lot of new content to a db, it has to be
optimized – re-indexed, etc.

6. Since PHP and WordPress is
not one of the fastest options, you might want to migrate to a different CMS.
For example you can have a web site done in Python using Django web framework
or use Ruby on Rails. Anything other than PHP will give you better performance.
However since you already have a lot of content, this is not really an option
for you.