For a 3x 1920x1080 Eyefinity setup, you will need the displayport adapter no matter what the GPU setup you have (unless you have a DP monitor).
With a single 5770, you could probably RUN it, but you'd be dropping your detail settings and stuff pretty low to get reasonable frame-rates. I would definitely crossfire to run eyefinity with anything less than a 5850. Most of the crossfire/eyefinity problems have been fixed with 10.2, and I think 10.3 will have a few more (10.3a beta was released today, and 10.3 WHQL will be released the 24th I think).
You will still need to plug all 3 displays into 1 card with crossfire. As far as portrait/landscape, its really just up to your personal preference. Some games also work better in portrait while some work better in landscape, bu you can change them back and forth if you want to.