In Debian / Ubuntu, simply installing one package will instantly make your web server faster. (That is, if you are serving up php!)
Here ya go:
sudo apt-get install php-apc
Next, edit the following file:
$ sudo nano /etc/php5/conf.d/apc.ini
And put in the following values. Increase the apc.shm_size as high as is available:
extension=apc.so apc.shm_size=64M apc.ttl=3600 apc.gc_ttl=3600 apc.mmap_file_mask=/tmp/apc.XXXXXX
scottlinux.com has php-apc installed with 256M of available apc memory!