I *think* that, should you buy a full copy of WHS 2011, you can reinstall the OS on top of itself (Server Recovery is the term), preserving your data (but not your settings, user accounts, etc.) However, in this case, backing up your data beforehand would be a good idea.
How to figure out what drive is the system drive? One way:
shut down, disconnect all drives but one. Boot the box. If you get an 'operating system not found' error, shut down, connect another drive, and boot. Repeat until you figure out which one has the OS on it.
How to figure out which drive has your data on it (and, back said data up)?
Yank them all. Get a USB drive adapter (Bytecc makes a really nice three-way one - best tool in my bag, got it from NewEgg.) Start connecting drives to another PC, see what's on there.