Two 4850's will be fine. The 4870's are the ones that suck a lot more power.
P.S.: Simple heat / fan fix (xml editing, easy) until RivaTuner is out for the 4800's (end of the month iirc):
Guru3D.com Forums - View Single Post - Radeon HD 4850 Heat issues
Can't wait for the 4870X2's though... sex: