roflcopter118 for 1 9800gx2 750 would be the least for that one card,for two I wouldn't trust it no matter how good the brand power supply was.first check how many amps the 9800gx2 uses and go from there.hope this help you out.
Each 9800gx2 uses 182w of power http://www.atomicmpc.com.au/forums.asp?s=2&c=7&t=9354
Depending on the rest of your system (Overclock?,RAM,HDs,DVD drives,fans, etc)
you should be fine with a quality 750w PSU. Double check your system with
jtt283s PSU calculator.
Most PSU manufacturers recommend you run their PSUs at 60% of rated load