Hello,
I am not exactly sure of the 590's, but I have 2X AMD 6950's from MSI in crossfire on my Z68-UD7. There is not very much room between the two PCI slots that will accept and run at the X16 spec, but if you have a good case with good air flow and most importantly what I found is using the fan schema in software to bump the fan speeds up as the temp rises helps a great deal. I use MSI afterburner, it has a fan control tab where you can essentially create a temp vs speed curve that fits your preferences.
Happy computing, and if I can be of any assistance, let me know!
Thanks,
Mike