Silvune :
Sorry, I didn't realise you were actually experiencing problems, I thought you were saying that MSI were saying you'd have those problems if you used your current PSU.
It really seems unlikely to me that your problem is a lack of power, but it is probably a good idea to check with a larger wattage unit anyway.
What motherboard do you have?
What case do you have?
How many games have you tested? It might be possible that the games don't like Crossfire mode.
Very obvious, but I want to double check that you installed all the necessary power plugs.
I have a Asus P7P55 WS SuperComputer mobo
I have the Antec DF-85 Case (the one with the red lights on it)
I've tested Just Cause 2, S.T.A.L.K.E.R. Call of Pripyat (but even with one card in, this game still has issues) and Battlefield: Bad Company 2. Now all three games are DX11 games.
Before, I had a 750 Corsair psu and 2 5850 video cards in crossxfire and I didn't have any issues. Until I bought the two 6970 lightning series cards.
I'm sure I've installed all the power plugs to the video card and to the mobo. Like I said, I'm waiting for a 1200 in win from newegg and I carefully read the comments and reviews on it from people who already bought it and been using it. Like I also said, if it still acting the same, I will return it back to newegg. I will let you know when I get it and see if it works!