A server is a concept very wide, please specify here what services must provide your server.
All or most part drivers you need included into the system's kernel so you need to recompile kernel with devices and options for your hardware configuration.
You may not have a problems with network cards except you use some different or specific nic's.
Whatever hardware you have will probably work just fine unless it's something unusual or very new. First step is just getting FreeBSD up and running. Getting that, and the basics of what you want, is relatively easy. Read the handbook on how to install FreeBSD and let us know when you are up/running and connected to the internet.
seems that you don't have experiences with running a *BSD system?! You should just do it, FreeBSD is a good choice. You have a complete and rock slid operating system with good handbook, clear website, friendly community...
Before you buy hardware, you should get familiar with their internals (chipsets) and check them against the hardware notes (e.g. http://www.freebsd.org/releases/7.0R/hardware.html. The shuttle box doesn't look that exotic. NIC is supported by the msk driver, the onboard graphic is supported with Intel driver under X, for sound FreeBSD has good support with the HDA driver... Just download a recent RELEASE (lets say 7.0, or 7.1 very soon) and give it a try. But don't expect that operating a server could be done without learning, no matter which OS you use.
I don't need sound or video.. I just need the VGA to run the FreeBSD command line.. And I want the network working..
Is there a user interface to monitor and set up servers or is it all manageed via the command line?