I was just buy another hard drive and install it in your system and then share it to the rest of your home network... But hard drives do fail more often then we like. Depending on what you are wanting to save, you might want to backup your data to either cd's or dvd's. I use 2 hard drives in Raid 1 configuration. That way if 1 drive were to fail then I still have all my data and OS intact and bootable. Please explain better on what you are trying to accomplish.
An NAS? I've considered getting one eventually. They are pretty neat and would probably be quite handy if your network contains a fair number of computers. However, they do have their problems.
If you look around, you tend to find most are limited to FAT32. I also read many problems about the cheaper ones with random disconnections and all. Also, some (like the Netgear NAS) require special software to access them, so that may deter you.
Oh yes, and as John said, you probably would want one that uses RAID. I figure if I could get one that used a RAID1 my files would be pretty safe. RAID0 doubles the likelihood of catastrophic failure, and of course using a single drive, well it could die at any time!
Well you can't expect anything too amazing over the network. Here's a decent list showing some NAS benchmarks.
http://www.smallnetbuilder.com/component/option,com_nas/Itemid,190
Plus realize many drives may be capped at ATA33 speeds, as many external USB devices are...