CPU recommendations for tc

It looks like I need way more CPU power than I expected.  I was hoping to just 
stream to external usb disk using P3 laptops and encode later, but then usb/disk 
IO becomes a bottleneck.  (I think it is the usb, might be the drives I am 
currently using.)

So I want to ask my local hardware guy to build me a box (5 actually) 
specifically for usb->transcode->local sata.  As long as I am doing this, I may 
as well get enough CPU to encode on the fly.

The hope is to capture:
/dev/video 1024x768 x24-bit @ 12 fps
/dev/dsp mono voice, so low bandwidth is fine.

I want the video saved in a lossless format.  (jpeg screenshots are ugly.)
If I can, I want the video compressed on the fly.
I want the audio saved in the same file, and hopefully with no sync issues.

Anyone know what kind of hardware will or won't do this?

Anyone have the math to support that I can't stream that much uncompressed A/V 
over a USB2 connection?

Fror more details: http://chipy.org/V4l2forPyCon

Carl K