My 3, ATI HD 3870 cards (on crossfire), hover around 70 to 80 degrees C after about 2 hours of moderate gameplay. This should be o.k, since each card has it's own fan.
Also, try keeping a window open and don't restrict the fan vents on you pc case. Another thing, is that if you get lots of dust in your computer, a lot of heat can build up!
Once I had crossfire on when one of the cards didn't have a fan and the heat went up to 120 degrees C. Luckily, I disabled crossfire after 10 minutes and the card is fine.