Hi,
Have you made some tests of Silverstripe on two version of PHP , that is 5.3 and 5.2.x?
I've made two openvz machine. One with Ubuntu Server and one with CentOS 5.4.
I see that CentOS is more responsive respect Ubuntu Server. I post, in attach, two pdf file with phpinfo.
From this info, What's the best situation or config?