Alternative to Windows Home Server

Right. I'm in the process of converting a Dell Optiplex GX-270 tower into a file / media server. P4, 2.8GHz, 512 MB RAM. It can handle up to 4 GB 400 MHz DIMM. As of now, I plan to use Windows Home Server. This is hard to say, but it appears from what I've read this may very well be the one piece of software Microsoft got right. However, I'd really prefer to not use WHS especially since the upcoming "Vail" update will whack Drive Extender. So, outside of Linux, is there any software out there that meets or beats what WHS can do. I'd prefer Open Source or freeware. Thank you.