Problem w Dual Monitors on ATI Radeon HD 4870 512 MB

Hi I'm having problems with Dual Monitors on ATI Radeon HD 4870 512 MB, Mac Pro 2009.

Setup: I'm running two 27" DELL U2713HM Displays at 2560x1440 one via direct DVI cable and the other one thru the Mini Display Port via Dual Link DVI adapter.

Now when I boot up the left screen displays the start-up apple as it should but then both screens go black and the right one (DVI connection) comes on while the left one stays black, goes into power safe mode. When I go into preferences > Displays I even get only one single display as if there were no multiple connected. It's however NOT a connection problem, since it works during the boot up sequence.

Any idea what might be wrong with my set-up? Do I need a new graphics card?