The 328 datasheet would be the best source for that kind of information. http://www.atmel.com/dyn/resources/prod_documents/doc8161.pdf
In the electrical section they show this data:
Parameter Condition Min. Typ.(2) Max. Units
ICC
Power Supply Current(1)
Active 1 MHz, VCC = 2V 0.2 0.5 mA
Active 4 MHz, VCC = 3V 1.2 2.5 mA
Active 8 MHz, VCC = 5V 4.1 9 mA
Idle 1 MHz, VCC = 2V 0.03 0.15 mA
Idle 4 MHz, VCC = 3V 0.18 0.7 mA
Idle 8 MHz, VCC = 5V 0.8 2.7 mA
Not sure how one would extrapolate that out to 20Mhz @ 5v, but you can certainly see the trend.