It varies from card to card. Best thing to do is test yourself.
I have a 3x5970 water cooled rigs. With memory at full speed temps are 30C over ambient and it pulls 1080W at the wall. With core overclocked to 900MHz (from 735MHz) and memory underclocked to 200MHz temps are 24C over ambient and it pulls 960 at the wall.
Best thing you can is experiment yourself.
Use a wall meter like Kill-a-watt, and set fan to static speed (say 70%) to record temps.
Set it to one setting, run miner for 3 hours (for good sample size). record temp, kWh used, and temp. Adjust memory clock and repeat.