Sounds like the OP has found a solution for their situation, but I wanted to add a couple things:
With two drives, OS X can use Mac OS Extended, and Windows can use NTFS (or whatever), and both will be happier.
If you're using it for networked storage, it doesn't much matter what format the actual drive is in--as long as the client computer can mount the particular type of share, it'll work fine for nearly anything.
For example, where I work we run an XServe, set up to share an HFS+ RAID1 drive set via both AFP and SMB (Windows). Both the Macs and the Windows boxes here can mount and use that shared volume just fine.
I've set up other businesses with a Windows 2003 server NTFS volume that was mounted from both Macs and PCs; in that case, the Macs were mounting it via the built-in SMB support, which also works just fine.
Mainly, though, I wanted to mention that I just set up exactly this sort of thing at my house; I bought a used G4 Gigabit tower (450MHz) for $100 off eBay, installed a copy of 10.4 I had on it, put a SATA card in it, and plugged in a couple of large, cheap SATA drives. Stuck it in the closet, connected it to my sound system, and I now use it as an in-house fileserver and my iTunes server. Once I get around to buying a copy of iLife, I'll also use it as my photo repository.
In my case, instead of using the server as backup, since I do a lot of work on both a laptop and desktop, I am working with files directly off of it. That way I really only need to have it do internal backups (from one of its drives to another), and I can easily access all my data whichever computer I happen to be using (or mount it via the internet when away from home).
With a gigabit hub (one of the new Airport base stations, in my case, which also acts as a print server) file access from any wired computer is not appreciably different from a local drive.
I do also run a small backup program on the machines that dumps a copy of the bookmarks and other prefs into a specified location on the server.
Quite nice, if that's the way you want to work. I personally prefer the centralized storage concept to just a backup server--keeps me from having to connect directly to another of my machines that has a file I need, or from having two copies of the same thing out of synch. But, that's just me and the way I work. (Also nice to have a dedicated in-house webserver for page testing, instead of running a local server on each computer.)
Were I to want to do ONLY backups, I'd probably have gone with a NAS.
