Most reviews/tests I see say under 300 Watts ish for full load...
Some 290 watts, others 105 Watts (Though I am sure thats more towards idle),
AnandTech: ATI Radeon HD 3870 X2: 2 GPUs 1 Card, A Return to the High End
Radeon HD 3870 X2 review - HIS
Legit Reviews - ATI Radeon HD 3870 X2 Video Card Review - Power Consumption and Conclusions
This one has the full system load with an X2 under 350 Watts.
I also looked at the Toms hardware review.
ATI Radeon HD 3870 X2 - Fastest Yet! | Tom's Hardware
And they say the total system power consumption with an X2 was right around 357 Watts, Furthermore they believe the card uses more energy than the AMD/ATI Stated 197 Watts, though I am still guessing it to be far less than 350 Watts...
So I still maintain that you can crossfire these cards on your current power supply and be fine.
So what I am really saying is, don't buy a new power supply unless you have to. Buy your crossfire setup, hook the cards up, run some games and 3Dmark06's, if your system does not shut off, die, or otherwise screw up. Then your power supply is fine..
It may also be interesting to see your results by monitering your Power supplies voltages with a digital multimeter during heavy GPU use, along with a Kil-o-watt Power meter attached at the wall to see total System wattage draw..