A few years ago I built a windows PC running Windows XP Pro as a file server that communicates to two iMacs running OSX Lion, a Windows Vista Laptop, Windows 7 Laptop and a couple iPhones, iPads and Apple TVs (Streaming from iTunes). As my iTunes library grows, and the number of digital photographs I take increases I'd like to build onto my server in a way that will utilize my existing hardware.
So other than the obvious add another harddrive, I can add one more, but I'd like to have 1 running Windows, 1 or 2 large HDDs for data, and sufficient HDDs to backup the data... my question is to all of you out there, what's the best option? I'd like to increase my storage exponentially, while being able to regularly backup data and still be accessible on my home network using all the devices I listed above.
Ideas?
that has been used as a

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic