Atmel ARM AT91SAM7S256 USB Question

Hi,
On our project, we are considering an ARM with a USB interface (Atmel
AT91SAM7S256). From looking at the data***, it appears that we need
an 18.432 MHz crystal for the USB to operate at full speed. Is this
correct? The data*** seems to be a bit ambiguous in this regard. I
looked at the schematic for Atmel's AT91SAM7S-EK evaluation board and
noticed that they are using an 18.432 MHz crystal, so I think I may be
on the right track. Will other value crystals (or digital clocks)
work? Could we drive the ARM at 36 MHz (this a required system clock
frequency for other components) with the ARM's main oscillator bypassed
and still run the USB at full speed?

Also, as I have never worked with an ARM before, does anyone have any
suggestions for tools? From reading this NG, it appears that the major
players are:

Rowley CrossWorks
IAR
Keil
GNU

What would be easiest for someone to get up and running "out of the
box" with? I have used Keil for 8051 development, WinAVR (GNU) for
Atmel ATMEGA work, and dabbled with IAR before.

Do these tools provide the start up code for launching the C main( )
function? Do any have utilities for configuring peripherals?

Any recommended RTOSs for the AT91SAM7S256?

Thanks in advance for any help.

Regards,
Doug

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