So. I added the card yesterday. Or well, I remembered wrong ; it was a 5350 not a 4 series card. But anyway. Adding the card resulted in windows crashing with stop code 7E. Had to uninstall catalyst 10.7 before I could boot in normal mode again. 10.8 driver caused my system to slow down a lot. Not sure if it's just 10.8 being broken or if it's my extra card, but I had to install the old 10.6 driver again in order to not drop down to 0-1fps in wow every couple seconds.
Anyway, with the 10.6 driver I can't play opengl games, but then that's been a problem with almost all 10 series drivers. Using 9.11 opengl driver for those games.
The crossfire however was NOT broken by adding the extra card. But it may very well have caused many of the other problems I've had yesterday.
Also interesting side note - I had a 3W higher power draw in wow than I had when the system bluescreened and went to the stop code. 491W while gaming, and 488W while bluescreening. In short - power management in the driver makes the game only consume 3 watts more than idle.