Help Needed on Crossfire X?

I hooked up 2 Radeon HD 6950 graphic cards via CrossfireX to play FSX with better graphics, but processing graphics became worse when I play FSX with those 2 graphic cards. Those graphics cards are relatively high-end stuff, and it's supposed to work better, but it doesn't. Possible cause of the issue that I thought was the power supply. I have a 660W, and it's not ideally big, but I thoght it's sufficient. I wonder if insufficient power supply is the case of the problem. Can someone advise?