The cheapest and easiest to configure/manage "Server" is Microsoft Small Business Server 2003. That's what I have in my home. In the past, I've seen Dell and Gateway packages for low-end boxes (including SBS 2003 Standard Edition) for as little as $600.
The actual SBS 2003 software can be acquired, in boxed Retail Version, for less than $500. The setup and maintenance is not unmanageable for a techie person who is willing to spend some time learning how Windows Server works. It IS, though, a whole different world than running an XP box.
Be aware that rack-mounted servers can be VERY loud. Especially the 1U units. These sound like a hurricane is passing by and are TOTALLY unsuitable for home use. The largest rack-mounted servers (4U and convertible) are probably fairly quiet, but I haven't used any.
If you want to buy a box, I'd recommend looking at the low-end Dell servers and installing a second SATA hard drive in RAID 1 configuration. You can get one of these, install some extra RAM and a RAID array, and put SBS 2003 on it for less than $1000 if you install the second drive yourself. Be SURE to buy the server with a floppy drive! You'll need it for the RAID array and for ASR Backups.
Store all your important files and documents on the Server (with the RAID 1 array) and back it up religiously. SBS has wizards to do full backups automatically. I'd recommend backing up to two USB hard drives. Swap them weekly and store the other away from the Server, in case of fire, flood, or theft.