i have a single ATI/AMD HD 4850 512mb and i am getting 45-90k/day in MWAH. i'd say that is pretty gravy. not to mention its 800 processors are over 150x as fast as my CPU at poem. this GPU uses 110 watts load. not too bad for something so powerful.
What kind of power supply do you have in that system?
How many additional 6 or 8 pin power connectors does that card take?