Thank You for the guide, but instead of spawn-cgi from lighttpd I would use php-fpm which uses resources much better than spawn-cgi. You can get php-fpm at THIS website and a simple guide how to install it, is HERE

The only problem is that there is no repo package with php-fpm so You have to compile php from source (well, it's not that hard).

Using a php-caching module can also help a lot. I prefer XCache but APC and eAccelerator would also cope quite well.

There is also one more thing, You shouldn't use nginx as root process (same with spawn-cgi). You can define the name of the user and group for the process (and it's children) in nginx.conf. Parent nginx process will be root but children will be working with the given user/group. Typical name for the user and group is www-data or nobody/nogroup.