Well, yes, driving two monitors requires more work from a GPU. But is 80 C normal, long-term-wise? I think not. You can get yourself badly burned at 80 deg, so why would this be good for a computer part, especially over 2 or 3 years? :-/
I remember I had a problem with a Mac Pro years ago. All the RAM slots were occupied, and iStat reported 70+ temperatures in these slots, even though the fans were doing their job. But I had a lot of crashes and kernel panics. Then I thought that such temperatures could not be helping. Then I installed a little helper app that enables you to set the minimum speed for any fan in you system. I just set the fans to 1200 rpms instead of the default 800, and everything began behaving smoothly from that point. Putting my hand to the back of the computer I could really feel the difference in temperature. And the modules' temperature went from 70-80 C to 35-45 C in about 5-10 minutes!!
Of course the fan noises was slightly more annoying, but I was willing to sacrifice a bit of my sanity for a stable computer. :-)
The app was named smcFanControl (http://www.eidac.de/). Works with Lion as of v 2.3. And free. There are of course other apps that would do the same, but this one has worked flawlessly for many years.
I think that Apple is setting it's temperature threshold limits a little too low, all for the sake of silence of operation...