I have been asked about this a few times and thought that I had posted about it but just made a comment on another blog.
Below are the steps that I did to install APC on my DV plan to allow many Wordpress installs to run without QoS alerts stopping the server.
Here is where I got APC from:
http://pecl.php.net/package/APC
Before you start this you should know that it is fairly involved to setup. This is far from a simple plugin. Having said that if you have time and some patience it is not that hard.
Make sure that you request to have the Developer Tools installed on your DV and also request your Root password as you will need it.
I used this version of Putty (it is really nice):
http://www.xs4all.nl/~whaa/putty/
For the install I did a mix of what is in the documentation and what I found here :
http://www.vbulletin.com/forum/showthread.php?t=165367
I had install wp-cache2 but removed it as it does not play nice with APC. Some people have a work around but for me just APC seems to be working out. Plus it is server wide and works for all PHP pages.