Philip:
"My comments regarding multiple instruction execution has no relation to DSP other than that's the platform where i learn to admire the technique."
And that's where it belongs, One of the most important features of the '51 in a REAL real time system is thet it is totally deterministic, that will be lost with multiple instruction execution.

"As to clock multipliers these are EMI dangerous unless your periphs are internal."
Not dangerous, they just do not help regarding EMI from external pins, but they do reduce the EMI from the oscillator or crystal.

Shankara:
"the effects of clock speed on powerconsumption/heating up and EMI problems."
Re heating/power consumption, it is right there in the datasheet for instance look at page 32 of
http://www-us9.semiconductors.philips.com/acrobat/d...0C32_1.pdf
Re EMI: EMI is increased by 2 factors: rise/fall times and frequency, in EXTREMELY EMI sensitive applications you would run at a low frequency and have capacitors on all signal lines to 'round' the pulses.

Have fun,

Erik