Nick, getting into a music server system is pretty complex, even when you know a lot about PC's. That's why I started a blog that walks through the various options in a manner that a non-tech can understand, but even techs can appreciate. The link is at the end of this email.
Why so complex?
1. You have to decide on the OS (operating system), then pick a PC.
2. You have to choose how to get the digital signal out of the PC to your system
3. Storage and back up issues
4. what to use as your player software
5. What formats to rip
And that is before you get into the audiophile issues. Here's my answer to your questions:
1) Can I add 2 additional harddrives and just use them for music only?
YES. I would recommend you consider external drives, which have their own power supply, and are easily portable. Firewire and USB 2.0 drives are reasonably priced, and you can easily use them with laptops or other PC's.
2) Do I need to add a bigger power supply for my pc?
MAYBE. No if you buy external drives, which have their own power supplies built in, maybe if you buy internal drives, which may require additional connectors and may tax your existing power supply.
3) Can I (or do I need to) control the various harddrives separately via a power switch of some sort?
External drives have a power switch on their case, which enables you to turn them off when not in use. Internal drives are powered by your PC's power supply, and boot up when you fire up your pc. You access them when you rip your music to the drives, or when you play the music contained on the drives.
4) If so, can the mouse/keyboard then control those music only harddrives?
NOT APPLICABLE. Your main hard drive will contain your operating system, which communicates with your mouse/keyboard. Your extra music drives simply hold music files, that's all.
For someone with an existing computer that is looking to add music server capabilities, you need only buy additional drives to hold the music and for backup, but use your existing computer and existing hard drives to store the playback and ripping software (and the Slim Server software you need for the SZB3).
http://musicserver.blogspot.com