X-Sendfile can dramatically increase the performance of delivering large files bye letting the httpd server take over in stead of php doing the job all the way.

Lighty:
http://blog.lighttpd.net/articles/2006/07/02/x-sendfile

Apache:
http://tn123.ath.cx/mod_xsendfile/

Nginx:
http://wiki.codemongers.com/NginxXSendfile