They are called Pull-Apart sensors, like the Sentrol 1500. You can see some at this link:
http://www.av-gad.com/products/sensors/AV-8105.html
And that USB cable is bad... OMG first of all, it is a $29 cable, not $95! You can get a 4-channel cable like that from www.Geeks.com for $60. Anyways, it is horrbile, buy the $10 USB camera before you spend $95 to turn a $100 CCTV cam into a junky USB1.1 camera!
Man, I have 3 extra 4-port DVR cards I would sell each for $35 before I saw you use that USB cable... I have 2 Swann and 1 DigiView cards sitting unused. If not, cool, but find yourself a deal on a real CCTV card or use USB cameras, but don't go the hybrid route or you will end up like me with $hundreds in equipment not being used.
In the $300 range you could get a 4-camera wireless 2.4g system, you could get a DVR card and 4 cameras, you have so many options. There is too much still to give you a definitve recommendation... How are you going to wire them, can you dedicate the PC the feeds are going to? What about power to run them? What about battery backup for them? Will any of them need to be mounted upside down, some camera's mounts don't flip well, is there direct sunlight...
So what I suggest is start with the DVR card and 1 camera. If you realize there are factors you missed, you can reassign that camera to a different position and get a more sutable one. By your 4th camera, you will have all of the features and locations covered... And go to Sam's, their $39 camera can not be beat... Color, Day/Night, 8-Leds, standard replacable lens, 100' cable, microphone... It will let you test all of your placements and provide your first camera, you can get some more high end ones once you know exactly what you need...
Lastly, if you are still thinking crazy though, consider like an ATI AIW video card where you can plug a CCTV camera straight into your video card... at least for your $100 you also get a cool TV card, plus you get to use a CCTV camera instead of just getting a USB converter.
Let me know if you want links to any of this. I will gladly let you log into my system and see my 8-channel system built using cheap parts..
Vaughn
Attached is a little picture of my in-wall camera system under the plasma, it is not much to look at, but I had it handy...