Interfacing again

I was just about to ask about interfacing when I saw the question
posted rescently in the archives.  However I still have the following
questions:

* What kind of datarate and cable length can I expect to get from SPI?
(Or how do I calculate it)

* Can I use a USB cable for SPI or UART?  I know it is non-standard,
but they are good, plentiful, pre-made cables that are known to handle
hi-speed.  I would like to get 100-400kbps.

* I2c does my datarate *but* I would like to use the hardware UART on
my avr chips if possible.  Only the large mega chips seem to have i2c
in hardware.

* Do I need to use interface chips to go distances of 5-6 feet? 
Assuming I only have my own devices on the bus, the logic level doesn't
matter on the line as long as both avr chips interpret 1 as 1 and 0 as
0.  Would like to minimize serial glue components.

People mentioned using port pins for chip select, but I can't run those
extra lines.  I am hoping that the four wires on the USB cable will be
enough for power, ground, clock, and serial data to connect my whole
system (strange, I know, but it's kind of a ropelight thing with
intelligent lights...)

So basically my question is has anyone abused RS-232 in this way? 
Which is to say running at 100kbps or more, using the hardware UART,
and going over a shielded cable.  If so how long were your cables?

What about interfacing with a PC?  I assume for that I would need an
RS-232 line driver to make sure the levels were correct.

Thanks,
-Jesse 

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