I use a Windows Home Server (WHS) to back up all my PCs (it finally supports Vista 64-bit machines), share videos/pictures/music to Media Center machines, and for remote access. No matter what you use, your disk space requirements will be based on the amount of data you need to back up and store. WHS makes this easy as you can easily add additional capacity as your needs grow.

However, if you need to also back up Macs/Linux boxes, you’re going to need to find another solution, or augment it with something else.