You don't need a good computer to run a web server at all. I would just get an old computer you don't use and make a deticated one. My ftp and web server runs fine on a 166. Also... there would be no point in building a nice computer to just act as a server because, unless you are on a connection other than cable/dsl your upload speed will be bottlencking everything anyway. Most cable companies have a max set at 30 k/sec for the upload speed. You want software such as...
http://www.schrags.com/modules.php?name=coppermine ??
http://dev.mysql.com/downloads/