It really depends on the card. This is an apples vs. oranges kinda question and can't be simply answered.
And my last ATI card got hotter under load but vented straight out of the case so it kept my whole system cooler. My new Nvidia card gets almost as hot but doesn't vent out of the case which is kind of a bummer. I installed a couple of new fans to address this and bring the temps back down.
Most videocards are designed to run extremely hot reliably so it's not really anything to worry about... except getting rid of that heat so that it doesn't affect your other components.
