I think that the answer is related to the software you will use to save videos.
If you save files as raw video, then you need a huge hard disk because RAW video has a very poor compression (or it is not compressed at all), but of you convert the video into any compressed format (xvid for example), then a powerful CPU and 2 GB RAM is the best option.
Of course, the video sizes are related also to the resolution and to the interval recorded (for example, if you save the video continuously then the file size will be huge, but if you save say 1 image every 2-3 seconds, the total size will be much shorter).
For what I know, the video card has no influence on the video encoding speed, but the CPU power is the most important thing. So I suggest to get 2 GB RAM as minimum and a powerful CPU.
Another thing you can consider is the power consumption. If you plan to run the PC 24/7, then a low consuming CPU is preferred. The recently introduced 45nm CPUs from Intel consumes 65W, vs the 95W of 65nm processors (with no overclocking of course).
I think that another thing you need is a NAS for backup, because a 24/7 running HDD has a shorter life than one running only 12 hours/day for example. So backing up videos every day can be useful