theksmith - impressive, highly technical build! Thanks for the nice feedback.
http://www.minidsp.com/ Lots of info and new devices available. rrray on the forum has a few posts on a nice low cost high quality USB interface and look up Soundman on the forum for PC software solutions.
The reason for the processor power is software compression/Encoding of the video. Record it RAW to the disk and processing power is minimal. This is where implementing a HW video encoder is worth it. However I only use the WEB cams for occasions where I want very high detail like an idiot in front or rear or scenery when travelling. For Black box recording with GPS positioning I use a dedicated Black Box Recorder. There is no way to guarantee that a Car-PC based system will survive a car crash whereas a Dedicated BB may have a chance.
You may not have noticed but to turn on both WEB cams and start recording when driving I just throw a switch on. To stop recording and switch cams off I turn the switch off. The cameras can be controlled via the rotary encoder between the seats and voice prompts tell me everything without taking my eyes off the read.
With 2 HI-DEF webcams streaming at 1280 X 720 @ 30 FPS to a 500GB 7,400 RPM hardened 2,5" Laptop drive on a SATA port there are absolutely no missed frames, Perfect smooth video for hours on end. You can easily consume over 10 GB per hour per camera with high resolution and again depends on the quality and type of the codec and your settings.