With 4800 heating problem, do not put more than ONE card or your rig will be a melted choclate
So much BS in this thread. The xml hack will get the temps down to the 50s on two 4850s. The 4850 is both power-efficient and potentially cool-running - if the fan is set to run at 50% or more duty cycle.
As for power supply wattage, when my Antec Quattro 850 failed (Manufacturing defect) I ran two overclocked HD 4850s, six drives, water cooling, and an overclocked Q6600 on an Asus Maximus with a 550 watt Silverstone Zeus supply for a week with no problems. The replacement Quattro is now back in the system and runs cool with the fan not audible during gaming on this rig.