I found this in the documentation for the Swann RedAlert system:
35,000 seconds (3,500 x 10) = 9.72 hours, so the math suggests a drive as small as 10GB could handle four cameras overnight where an 80GB drive should be able to store almost four days of continuous video from the same cameras. I have yet to get into the DVR4-Net's resolution options that would likely affect that, of course, but yes, I do plan to use a motion sensor so the system will only record when necessary.
I gave one of the Swann cameras a try early this morning and it did fairly well. It has only 12 IR LEDs, but but it looks like it will do just fine in my carport. However, I now have to wait for some BNC-to-RCA adapters to arrive before I can connect anything to the DVR4-Net recorder.