Sounds like you'd want some software that also had movement detection, that could automate that without you having to figure out whenabouts you might have to change the capture rate. I'm interested in this sort of thing, too. I've not found anything that would work with my system to capture images at will, even though my webcam does actually work with things like Ekiga.
Look into ZoneMinder: http://www.zoneminder.com/
