How to compare computing power of microcontrollers?

Hi - I'm trying to choose a microcontroller for a project I'm working
on. Originally I had been thinking about using a 20Mhz Atmel AVR, such
as an ATMEGA168, but I became worried about the raw computing power of
the AVR, as one of the main roles of this chip will be to do alot of 16
bit multiplication and trig at a fairly high speed. So I've been
looking at using an ARM - specifically the Atmel AT91SAM7A3 looks very
interesting to me. It's clocked 3 times as fast as the ATMEGA168, but I
expect that isn't a good way at all to compare it's ability to handle
this task to the AVR's. Is there any good way to make a comparison for
this application without benchmarking the chips? Thanks,

-Michael J. Noone

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