I was going to mention that and didn't. For a few bucks I would add another drive and use it as backup. Unless someone is really into saving countless GB of video a 1 TB drive is a pretty big drive. Adding another 1 TB isn't all that much money. Both Vista and Windows 7 offer an OK backup feature that allows easy backup to another drive. Anymore HDD space is pretty cheap.
The battery and additional PSU were something I did as a spoof years ago using the wife's old system. It was done for a PC Magazine forum discussion about the then new GPUs power demands. I still had the images.
While everything here is on pretty large UPS units the entire house is on an emergency backup generator so at most I only need about 10 seconds of UPS support and yes, every few years battery shopping is a *****!
Ron