SPI questions

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I have to start a new design using nodes connected via SPI (due some external reasons I can not use another bus) and have a few questions:

1. Wich is the maximium practical distance I can transmit data with SPI at rates from 2 to 4 Mbits/s? I want to use flat cable like the one that comes with the STK500 for the In System Programming via SPI, but longer.
2. I found that some small AVR parts (i.e. Tiny) does not have the SS signal in their SPI port. Is this signal essential in order to get the SPI working? For example, I want to use 10 ATMega88 connected with SPI but without connecting the SS line at all.
3. There are four "SPI Modes" (0 to 3) and I do not know for wich applications should I use wich mode.

Thanks in advance for any help you can bring me!