If its an option, add some intake fans to the side of your case. Blowing cool outside-of-the-case air directly onto the video card will ensure its keeping as cool as can be.
90degrees sounds fairly high, although i dont know too much about 590's so maybe this is the norm - especially since its a stock card.
EDIT: Thinking about it, if its effectively 2 cards in SLI within 1 card, then 90 degrees is probably about right.
EDIT2: Just found this on another thread "These temps are within specs. The card is rated to handle up to 105c. Normal operating temps are 35-50 at idle depending on cooling of case and air flow."