It should work seeing as the ASUS M4A89GTD PRO USB3 has two 16x PCI-e slots. However I'd go for a single HD 5850, the performance may be slightly less, although you can expand by putting another HD 5850 later on.
If your choice is 1 5850 or 2 5770's the choice is easy...5850 all the way. It's ALWAYS better to buy the higher performing card, and then someday crossfire it (if you ever need to), rather than buying the lower performing card and crossfiring it right away. I do believe the 5770's in crossfire WILL slightly outperform a single 5850, usually, but there are some issues with crossfire / sli that are avoided with a single card, and it's also the more "future proof" choice, as I said. Plus, unless you are gaming at a 2560X1600 resolution, 1 5850 will be plenty strong enough for anything you can throw at it.
I have a pair of 5850's, the width isn't the issue, nor them touching each other. I have them sitting right next to each other in a crossfire setup, they don't touch, and temp's aren't an issue in my modestly cooled case (decent airflow but nothing spectacular), because these cards stay VERY cool for the power they put out. The biggest issue I had was installing the *@ power connectors. For some reason, the folks at ATI, in all their wisdom, decided to put the power connectors at the END of the cards, rather than the side. This made connecting them very awkward in my case. So make sure you have room at the end of your case when you install a 5850. I'm pretty sure a 5770 uses this type of connection too though, however.