You know that computer you put in the corner the last time you bought a new compter?
Use it. Im sure it will be fine for a board your size.
There are 2ish routes as far as software goes.
1st is Linux, Something like Fedora is free to use & has good support. But Linux can be difficult to learn at first.
2nd is Windows, This is where the "ish" comes to play. Windows XP is capible of being a 1 domain server. So if you only plan on running 1 site off the machine it may be the most "cost effective" method, but it has its limitations. Microsoft has a server OS called Microsoft Windows Server 2003, If you can afford to buy an OS, and are not firmilar with Linux, i would suggest this route.
Try googling home server, there is tons of info out there...
Here is one i found quickly:
http://www.htmlgoodies.com/beyond/we...le.php/3499361