Alienware m17 video card upgrade from dual ATI HD4870 to dual ATI HD5870


Apr 26, 2015
Hi. I have an Alienware M17x R2 with dual HD4870, which has pretty decent performance but, as you can imagine, with the 4870s it can't play many modern games requiring recent DX versions.
I happen to have a couple of working ATI HD5870 cards which are supposedly compatible with this machine, and when I install them and connect them with the crossfire cable, everything fits perfectly, as if everything was designed to work together.
But then I power up the laptop and nothing happens. Keyboard and buttons light up, but screen never comes up, and the computer powers down after about ten seconds.
What am I missing?