I've talked before on this blog and in the podcast about my Windows Home Server. It's probably one of the best things Microsoft has ever put out. You can read my review of its features here. To sum up: it backs up your computers, creates a central repository for all of your data, and gives you secure remote Internet access to all of it.
And one more thing to like about it is that HP's version (the MediaSmart EX470 server) is on sale for $299 with a little bit of financial gymnastics (but totally legal, I assure you). This machine comes with one 500GB drive and slots for three more. If you want to be extra geeky, put four 1TB drives in there and you'll have storage to last a lifetime (YMMV).
A big shout-out for the excellent blog Home Server Hacks for the tip on this. I may have to spring for one myself at this price.