Note: I'm not trying to evaluate linux versus FreeBSD at all, just thinking about some of the benefits of the FreeBSD and CentOS distributions.
After setting up a CentOS virtual private server, I found it used very little memory. I found that super cool, and so I'm revisiting FreeBSD to explore the limitations of its memory capabilities.
The smallest FreeBSD I've used would have to be m0n0wall, and that needed at least 64 megabytes of RAM - but that included mini-httpd and PHP.
I bet it could go much smaller. I wish there were more FreeBSD VPS hosting companies out there.
I've read that NetBSD uses very little memory, so I'm going to try that out. It also has been reported to have good Xen support, so I'm going to try that out too.