I'd recommend that you look at bacula. I've used many backup systems (both commercial and open source) over the years and bacula is by far the best of them overall, is under active development, robust, scalable, and has optional commercial support if needed.
For CentOS 5.x, there are not-the-most-recent-version RPMs available from the bacula site or you can build from source. As of RHEL6/CentOS6, bacula comes with the base operating system (as usual, also not the most recent version but quite a bit newer than the 5.x RPMs).
The bacula *server(s)* must run on a UNIX machine. Bacula clients are available for many different UNIX variants as well as Mac and Windows.
There is a learning curve for setting it up but if you've ever used any other enterprise-grade backup product it shouldn't be overwhelming.
Devin