Excellent advice here. Basically, if you don't play to run a graphical environment on the server you can save quite a bit by going with a very low-cost video card (even an integrated one).
For a server, definitely implement at least some basic software RAID so a simple, single hard drive failure doesn't stop you cold.
(That's still no excuse for a good backup, though!
)
CentOS is a great server OS; not only is it free, but it's based on Red Hat Enterprise, so there's plenty of documentation and howto's on getting started and configuration.
Most of all, though, have fun!