Despite the fact that the work I do is not top secret, nor do I have any enemies, I have always felt safer and better protected with the presence of security cameras. You just never know what you might pick up. Plus, I often work late at night and my office is next to a bar, and since my car sits outside, I like to be able to keep an eye on it at all times.
I use a recording program from DeskShare called Security Monitor Pro that costs $90. This software allows me to start recording on motion detection. It takes video clips, still picture, and can even email me the media or publish to an ftp server. I record my cameras at 10fps, which allows me to see enough action so I don't miss anything, while not overloading my CPUs.
Then at the core of it all is a dedicated Acer Aspire. I picked up this $500 computer from Fry's. It's loaded with an Intel i3 at 3.06GHz with 6GB of RAM and a 1TB hard drive. It's amazing how inexpensive technology is these days. I previously had a lower-end Dell recording, but it only had a single core and it choked when I tried to view and record two near-HD streams.
So for as little as $700, I have a security system that is computer-based, and actually records higher quality video than typical dedicated prosumer security rigs.
Like I said, this is a little over the top for my needs, but it's nice to be able to see my surroundings. If you're looking to set up a basic security rig, I definitely recommend going this route. Like I said, I've toyed with lots of possibilities, and since this system records high quality video with great features that lets me see what's going on when I'm away from the office, there are really no downsides.