Actually I suggest you to test software before buying it as well as a hard drive. Depending on your computer performance you may wanna record videos to different formats like MPEG-2, WMV, MPEG-4. You may also wanna consider a web-cam that can compress videos to MPEG-4 format using hardware not software.

You might also consider web-cam recording software like Active Web-cam that has motion detection. The quality of such programs can vary significantly. But really goods ones in most cases record videos only when something happens. They can also send you an alert via e-mail or by some other methods.